A few hours after dawn on 8 May 2015, the morning after his unexpected victory in the general election, David Cameron delivered a celebratory speech to the jubilant staff of Conservative campaign headquarters, at 4 Matthew Parker Street, Westminster. “I’m not an old man but I remember casting a vote in 1987 and that was a great victory,” he said. “I remember 2010, achieving that dream of getting Labour out and getting the Tories back in, and that was amazing. But I think this is the sweetest victory of them all.”

The assembled Tory campaign staffers cheered and whistled as Cameron declared: “We are on the brink of something so exciting.” The election result would indeed change British politics, although not in the way that Cameron intended: the obliteration of the Conservatives’ Liberal Democrat coalition partners cleared the way for the referendum that set Britain on a path to leave the EU and ended Cameron’s political career. As a result, Theresa May is now the prime minister, while Cameron is on a speaking tour of US universities and George Osborne is moonlighting as a newspaper editor.

Until recently, Britain thought it knew how the Conservative party had defied expectations to win the election. After the initial shock that predictions of a hung parliament had proved incorrect, a new narrative was soon established. Commentators explained that the Tories had prevailed by successfully emphasising the threat of a Labour coalition with the SNP and deploying the “pumped-up” prime minister for a spurt of decisive last-minute campaigning. Several newspapers reported that the Tories had spent less to win their 12-seat majority in 2015 than they did to win 24 fewer seats in 2010.

In truth, the victorious Conservative campaign was the most complex ever mounted in Britain, run by two of the world’s most successful campaign consultants. Warehouses of telephone pollsters were put to work for a year before the election, their task to track the views of undecided voters in key marginal seats. The party also distributed thousands of detailed surveys to voters in marginals, and merged all this polling data with information from electoral rolls and commercial market research to produce the most comprehensive picture yet of who might be persuaded to vote Conservative.

Armed with an unprecedented level of detail, the Conservatives began distributing leaflets and letters that directly addressed the hopes and fears of their target voters. And in the final weeks of the campaign, shock troops of volunteers were dispatched to the doorsteps of undecided voters with a mission to persuade and cajole on the party’s behalf. In the most high-profile fight, an elite squad of strategists moved from the London HQ to Kent, where the Ukip leader Nigel Farage was making his bid for parliament.

If the sophistication of the 2015 campaign was not widely known, that was by design: the Conservative Home website, a meeting place for party loyalists, called the victory a “stealth win”. But over the last few months, another story has emerged – an account that is told in a paper trail of hotel bills, emails and witness statements that has led to a year-long investigation by the Electoral Commission and the police.

The startling evidence, first unearthed by Channel 4 News and confirmed in a condemnatory report released last week by the Electoral Commission – the independent body that oversees election law and regulates political finance in the UK – suggests that the Conservative party gained an advantage by breaching election spending laws during the 2015 election. This allowed the party to send its most dedicated volunteers into key seats, in which data had identified specific voters whose turnout could swing the contest. Some of this spending was not properly declared, and some of it was entirely off the books. The sums involved are deceptively small, but the impact may have been decisive.

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At present, up to 20 sitting Conservative MPs are the subject of criminal investigation by 16 police forces. If any of the candidates are charged and found guilty of an election offence, they could be barred from political office for three years or spend up to a year in prison. The whole case is unprecedented: this is the largest number of MPs ever to be investigated for violations of electoral law. In the past, cases of alleged election fraud have usually focused on a single MP. This time, there are so many cases that police forces across England have taken the unusual step of coordinating their investigations.

The release of last week’s 38-page Electoral Commission report produced a minor political earthquake: as a result of the biggest investigation the commission has ever undertaken, it levied its largest-ever fine against the Conservative party and referred the case of the party’s treasurer, Simon Day, to the Metropolitan police for further criminal investigation. “There was a realistic prospect,” the report said, that the undeclared spending by the party had “enabled its candidates to gain a financial advantage over opponents.”

The party’s response to the report has been dismissive from the very start. During their investigation, the Electoral Commission was forced to file papers with the high court, demanding that the Conservative party disclose information about its election campaign, after the party had failed to fully comply with their requests for information for three months. Since the report was published, Conservative ministers and spokesmen have pointed out that the commission found only “a series of administrative errors” and that other parties have been fined for their activity in the 2015 election too. Conservatives also say that the missing money identified by the commission represents just 0.6% of the total spent by the party during the 2015 election.

It is true that the sums involved in this case are small: the Electoral Commission’s highest-ever fine turns out to be just £70,000, and it has been applied to punish undeclared and misdeclared Conservative spending totalling just £250,000. Most reports on the commission’s findings have echoed this defence, allowing that some criminal charges may indeed be filed, while overlooking the impact of the overspending on the result.

But British elections are designed to be cheap. Laws that date back to the 1880s limit campaign spending precisely so that people of all backgrounds, and not only the wealthy, have a fair chance to compete for votes. And if that egalitarian principle enhances our political culture, it has another less obvious consequence: even small sums of additional, illegal money, if shrewdly spent, can make a huge difference to results.

Thanks to the Electoral Commission report, we now know that some of the Conservative party’s central spending did benefit MPs in the tightest races, but it was not declared. It is possible even that this money helped to secure the victories from which the Conservative majority was derived. Slowly, a chilling prospect emerges that British politics, our relationship with Europe and the future of our economy, were all transformed following a contest that wasn’t a fair fight.

The Conservatives’ election worries were never financial. By the end of 2014, newspapers reported that the party had raised substantially more money than its rivals, assembling a £78m “war chest” that would allow it to “funnel huge amounts of cash into key seats”, according to the Observer. The campaign would be constrained only by two factors: the legal spending limits for each candidate and the number of volunteers the party could recruit to take its message to voters.

In fact, the scandal in which so many MPs now find themselves embroiled concerns precisely those limits. The spending that has been found to be in violation by the Electoral Commission was used to bring Conservative campaigners into the tightest marginal election battles. Separately, multiple police investigations are examining whether individual candidates and their election agents broke the law.

It is difficult to understand the election expenses scandal without understanding the election strategy that had been unveiled three years before the vote. At a closed session on the first day of the 2012 Conservative conference, the party’s campaign director, Stephen Gilbert, laid out a plan that would come to be known as the 40/40 strategy. For the 2015 election, the party would focus single-mindedly on holding 40 marginal seats and winning another 40. Candidates for these seats would be selected early, and full-time campaign managers – heavily subsidised by Conservative campaign headquarters (CCHQ) – would be appointed in every 40/40 seat.

The 40/40 campaign would be centrally controlled and would require two ingredients. The first was detailed information about every potential Conservative voter in each of the marginal seats. The second was a field team capable of making contact with them and persuading them to vote Tory.

To put the plan into action, the party turned to two men who have helped reshape the way elections are fought. The first, the Australian political strategist Lynton Crosby, had overseen the Tories’ 2005 general election campaign and Boris Johnson’s two victories in London mayoral elections.

Crosby’s notoriety made him the subject of considerable press attention – but the second man behind the Conservative campaign may have been even more important. This was the American strategist Jim Messina, who was hired as a strategy adviser in August 2013. Senior Conservative staff had been awestruck by Barack Obama’s comfortable victories in the 2008 and 2012 presidential elections, crediting their relentless focus on data to Messina.

British elections are designed to be cheap: even small sums of additional money can make a huge difference to results

Using vast databases, commercial market research, complex questionnaires and phone banks, Messina had been able to map the fears and desires of swing voters, and design highly personalised messaging that would appeal to them. The Conservatives hired him to perform the same magic in Britain. To do so, Messina used commercial call centres to track the views of between 1,000 and 2,000 voters in all 80 of the seats targeted by the 40/40 strategy.

This data was crucial to the Conservative campaign: it determined which voters the party needed to contact and which messages they would hear. This began with direct mail – personally addressed to voters in each target seat, who were divided into 40 different categories, with a slightly different message for each one.

But the big-data strategy requires more than leaflets: once you have identified the voters who might be persuaded to switch, and fine-tuned what message to give them, you have to send campaigners to actually knock on their doors and urge them to go to the polls on election day. This requires an army of volunteers, spread across dozens of constituencies. It fell to the party’s co-chairman, Grant Shapps, to establish the necessary volunteer outreach program, which was dubbed Team2015.

Shapps had begun sending out recruitment emails to the party’s mailing list in the summer of 2013, hoping to build a centrally controlled base of activists who could be deployed to marginal constituencies. CCHQ demanded that Team2015 coordinators be established in every swing seat. It was an uphill struggle. Rallying enthusiastic volunteers to David Cameron’s cause turned out to be a harder task than attracting Obama supporters had been.

Facebook Twitter Pinterest ‘Under David Cameron’s leadership, the number of party members had further depleted, halving to fewer than 150,000.’ Photograph: Peter Nicholls/PA

Conservative membership had been in long-term decline from a peak of 2.8 million in 1952. Under David Cameron’s leadership, the number of party members had further depleted, halving to fewer than 150,000. Those remaining members tended to be older and less active – not the dynamic door-knocking volunteers that Team2015 wanted to recruit. While some local Conservative associations reported new members, most described numbers as “hit and miss”. One seat’s early Team2015 report records: “[Team2015] invited to party with MP – no one turned up!”

In some marginal seats, Team2015 was almost nonexistent. One campaign manager recalls: “Trying to get members to volunteer was practically impossible, so Team2015 volunteers were even worse. People would put their names down, generally via CCHQ, who would then pass the person’s details to the local campaign manager but, in my case, when I tried to contact them I never got any volunteers.”

As the election drew nearer, Shapps made upbeat reports on the growing volunteer force. But, according to Conservative Home, the party’s records indicate that only about 15,000 people ever turned up to campaign, and fewer than that did so regularly.

There was, however, another team at work. Unsupervised by CCHQ to start with, it would later be adopted as a critical element in the party’s “ground war” since – unlike Team2015 – it had managed to deliver platoons of committed Conservative activists to the places that needed them most in a series of crucial byelections the year before. It was called RoadTrip.

RoadTrip2015 was the brainchild of Mark Clarke, who would become infamous after the election as “the Tatler Tory”, pilloried in the press over accusations that he bullied a young Conservative who later killed himself, and made unwanted sexual advances towards female members of the party – allegations he has always denied. But in 2014, as a failed parliamentary candidate desperate to get back into the party’s good graces, he launched a grassroots volunteer scheme that sent party members into marginal seats to distribute leaflets, knock on doors, and work the voters.

RoadTrip2015’s work began with a March 2014 trip to Cannock Chase, a West Midlands Labour marginal where 50 volunteers battled through a hailstorm to the doorsteps of swing voters. In the months that followed there were trips to Harlow, Chester and Cheadle. In Enfield, Team2015 marshalled 130 volunteers and party co-chair Grant Shapps attended too. But what put the scheme on the map, and drew the admiration of Conservative commentators and MPs, was the Newark bylection in early June 2014.

On 31 May, the Saturday before the byelection, Clarke successfully marshalled 500 volunteers to Nottinghamshire to campaign for the Conservative candidate, Robert Jenrick. Clarke posted his invitation across social media and on the Conservative Home website: “Join us, Grant Shapps and the hundreds of people signed up this Saturday to come to Newark. Afterwards, join Eric Pickles for the inaugural annual RoadTrip2015 dinner (a free curry) in nearby Nottingham. We will take care of your travel from cities like London, Manchester, Birmingham, Bristol and York.”

The Newark campaign was the first major stress test for the Conservatives’ parliamentary election team. By polling day, 5 June, they were feeling intense pressure from Ukip, which had triumphed in the European elections two weeks earlier – showing they were more than capable of stealing support away from the Conservatives.

Before Clarke’s RoadTrip arrived in Newark, a small team of senior Conservative staff – including Stephen Gilbert and a “campaign specialist” named Marion Little – had quietly taken position on the outskirts of the town at the Kelham House country manor hotel. In Newark itself, many more junior party employees – some of them campaign managers from other 40/40 seats – worked from temporary offices during the day and, at night, stayed in a Premier Inn.

The well-resourced Tory campaign turned out to be decisive and Robert Jenrick was returned with a 7,403 majority – rather smaller than his predecessor, but still substantial. But, on the evening of the count an exasperated Nigel Farage, interviewed by Channel 4 News political correspondent Michael Crick, raised the first concerns about Conservative election expenses – which, he suggested, might have breached the £100,000 limit for campaign spending in a byelection.

“Given the number of paid professional people from the Conservative party here, it is difficult to believe that their returns are going to come in below the figure,” Farage said, referring to the documents every candidate must file to detail their campaign costs. “I’d love to see what their returns are. Because it seems to me the scale of the campaign they fought here is so vast … There will certainly be some questions.”

The rules in a byelection contest are simple. All costs incurred in promoting the candidate in parliamentary elections – advertising, staff costs, unsolicited leaflets and letters, transport for campaigners, hotels that volunteers do not pay for themselves, and administrative costs such as phone bills and stationery – must be declared. Deliberate overspending can be a criminal offence, and it may also lead to an election being declared void.

Robert Jenrick’s campaign in Newark had declared expenses of £96,191. But the Electoral Commission later found that his return did not include the hotel bills for 54 nights of accommodation for senior Conservative staff, or 125 nights of hotel rooms for junior Conservative staff at the Premier Inn. Those costs totalled more than £10,000; had they been declared, the campaign would have breached the spending limits. Farage had been correct. (When questioned by Channel 4 News in 2016, Jenrick denied all wrongdoing. In response to questions about by-election hotel expenses, the party responded that “all byelection spending has been correctly recorded in accordance with the law”.)

At the time, however, these details remained unknown – and Channel 4 News reporters did not discover the undeclared hotel bills until long after the one-year time limit for the imnvestigation and prosecution of election crimes had passed. As a result, there was little attention to increasing Conservative spending in two more crucial byelections.

In October 2014, another huge team of Conservatives descended on Clacton-on-Sea, where Douglas Carswell had defected from the Conservatives to stand as a Ukip candidate. Again, hotels were booked for visiting campaign staff, and a return of £84,049 was filed – which did not mention all the party’s hotel costs of 290 nights at the Lifehouse Spa & Hotel, and 71 nights at the Premier Inn, worth at least £22,000. Had they been declared, the overspending would have been more than £8,000.

In Rochester and Strood, where the defection to Ukip of yet another Tory candidate, Mark Reckless, prompted another byelection in November 2014, the Conservatives could have breached the spending limit by a far larger amount – more than £51,096. As detailed in the Electoral Commission report, their candidate did not declare hotel costs of at least £54,304 against expenses of £96,793. The Conservatives still lost both contests. (Neither of the Conservative candidates responded to requests for comment. The party replied on their behalf that all spending was filed in accordance with the law.)

In these byelections, RoadTrip2015 – which was now supported by CCHQ and endorsed by Shapps – became an increasingly important influence. When the campaign launched a Facebook page advertising for a “Clacton Volunteer Force”, 1,300 people signed up to take part. In Rochester and Strood, it offered volunteers who turned out on Saturday 8 November “FREE transport there and back, FREE drinks and access to the FREE RoadTrip2015 Disraeli Dinner with a very special guest speaker!” The guest speaker was Theresa May, who was filmed celebrating with volunteers. She said: “What you do matters so much because, although what the politicians do has got a role to play, in terms of election campaigning, it’s the people who go out on the doorsteps, who knock on those doors, who make those telephone calls, who put those leaflets through the door, that make a real difference to the results we have.”

By the time of the 2015 general election, the tactics that the party had used to saturate all three byelection constituencies with activists and workers would all come together: there would be more buses of volunteers, more undeclared hotel bookings, and more senior advisers moved out of London into crucial seats. But this time, it would be discovered.

Today, two pieces of rather antiquated legislation exist to tame the influence of money on our elections. The first law governs spending by constituency candidates in the run-up to a general election during two time periods: the “long campaign” runs from about six months before polling day until parliament is dissolved; what follows is the “short campaign”, a final frenzied push for votes that lasted for 38 days in 2015.

The spending limits in each period are tight, with exact values depending on the type of constituency (borough or county) and the number of voters. For the “long campaign” in 2015, the totals were typically around £35,000 to £45,000. While in the short campaign, the most crucial campaign period, the limits were tighter still, set at £8,700 plus 6p or 9p per elector, giving a limit of around £10,000 to £16,000.

The limits are low, theoretically allowing as many people as possible to mount a viable campaign for election. Any costs incurred promoting the candidate in the constituency – from advertising, administration and public meetings, to party-paid transport for campaigners, staff costs and accommodation – must be honestly declared. At the end of the campaign, every penny spent must be declared in an official spending return submitted soon after the end of the campaign. Each spending return includes a declaration that certifies it is “complete and accurate … as required by law”. This must be signed by both the candidate and their election agent – a member of the local party that they appoint to manage their spending. Failing to declare spending, and spending over the limit, are criminal offences.

The second election spending law applies to political parties, and sets much higher limits for their spending on national campaigning during a specified period – roughly a year – before the election. The precise limit is derived by multiplying the number of constituencies being contested by £30,000. For the Conservatives in 2015, this gave the party a national limit of £18.9m to spend promoting David Cameron and his plan for the country through advertisements, billboards and direct mail. As it turned out, the party ended up declaring a figure well below the limit – around £15.6m. It is the responsibility of the national party treasurers to ensure that these national returns are correct, and again they commit an offence if they are found not to be.

Of course, the existence of two different laws setting out two different spending limits – one for local spending and one for national spending – is a source of potential confusion. In the real world of campaigning, there are bound to be expenses that do not fit neatly into one category or the other. For example, leaflets may contain a national message on one page – promoting the party’s leader or policies – and a local message, from the constituency candidate, on another page. When this happens, both the party and the candidates are required to make an “honest assessment”, in the words of law, about how much of the cost of the leaflet should be declared on both returns, before “splitting” the value accordingly. To aid transparency, election material must, by law, carry an “imprint” that shows whether it was produced for the local candidate or for the national campaign.

But the presence of two separate spending laws also presents an opportunity for abuse. Much of the scandal surrounding the Conservative party’s 2015 election spending relates to evidence that suggests spending declared as “national” – where limits are much higher – was, in reality, used to promote local candidates, who face much tighter spending limits.

In fact, it is the enormous difference between the national limits, in the millions, and the local limits, in the tens of thousands, that makes these allegations so significant. Even small amounts of candidate overspending – easily buried in the multimillion-pound national accounts – could have a significant impact on a local campaign, and even shift the result.

Following Ukip’s triumph in the Clacton and Rochester byelections in late 2014, the Conservative campaign faced a miserable winter. Labour led the polls for a few months, and by April 2015, pollsters and pundits were predicting a hung parliament.

The Conservatives made two moves that helped to turn the tables. The first was a new message – to stoke fear that without a clear Conservative majority, Britain would be run by a coalition between Labour and the Scottish National Party.

The second was a new tactic, based on RoadTrip2015. Mark Clarke’s day-long campaign events in the run-up to the general election had given the Conservatives a taste of what the party desperately needed – enthusiastic volunteers knocking on doors in areas that mattered. Historically, Labour had better form bringing activists into marginal battlegrounds, largely thanks to its more active membership drawn from the unions. The Conservative party, with its dwindling and increasingly inactive membership, often found it had no response.

The Conservative party insists that the BattleBus was only intended to conduct national campaigning

But a new plan grew from the seeds of RoadTrip, one that involved busloads of activists and block-booked hotel rooms. BattleBus2015 would send a fleet of coaches to three regions of the UK – the south-west, the Midlands and the north – for the final 10 days of the election campaign. These mobile units, each with around 40 party activists, would stay in hotels in each region, from where they would be loaded onto coaches and driven into different marginals to campaign each day. This would allow the party to flood 29 key seats with much-needed support: nine in the south-west, 10 in the Midlands and 10 in the north.

Receipts for the hotels and coaches, obtained later by Channel 4 News, would prove the operation was expensive. The Electoral Commission later calculated that the BattleBus operation cost £102,483, which works out to around £3,500 for each seat it visited. But while the national party could easily absorb the cost before hitting its spending cap, many of the local candidates were already cutting it fine. If they had to declare the extra costs associated with bringing in more campaigners, the majority would breach the limit.

In the event, £38,996 of the BattleBus costs were declared on the Conservative party’s national return, while the other £63,487, which included the hotels used by volunteers, was not declared at all. The Conservative party put this down to “human error”.

None of the 29 candidates visited by BattleBus declared any of its costs. Whether this should be categorised as national or local spending depends on what the activists did: if they promoted local candidates, even part of the time, then at least some costs associated in bringing them to the constituency should have been declared locally.

The Conservative party insists that BattleBus was only intended to conduct national campaigning. The Electoral Commission report states that it “has found no evidence to suggest that the party had funded the BattleBus2015 campaign with the intention that it would promote or procure the electoral success of candidates”. But, the report continues, “coaches of activists were transported to marginal constituencies to campaign alongside or in close proximity to local campaigners,” and “it is apparent that candidate campaigning did take place during the BattleBus2015 campaign”. It adds that, in the commission’s view, a proportion of the costs should have been declared in candidate campaign filings, “casting doubt” on whether these candidate spending returns were accurate.

The Conservative party has responded to these allegations by insisting that BattleBus volunteers did not promote local candidates. But on Twitter, in the weeks before the election, the BattleBus activists hailed their own efforts to win over voters for specific candidates. On 2 May, one volunteer wrote: “1,300 voters talked to on the doorstep in Amber Valley today for @VoteNigelMills!”. Another posted: “Nice homes in the beautiful Amber Valley – great reaction on the doorsteps in support of Nigel Mills.”

Photographs posted on social media add to the layers of evidence. One young female activist is pictured on a doorstep holding a leaflet bearing the name of Nigel Mills. In the north, a group of activists in Sherwood were photographed holding calling cards for the candidate Mark Spencer, carrying his name and image, and the words: “I called by today with my local team to hear your views.” Channel 4 News has spoken to a handful of volunteers who say their time on the BattleBus involved local campaigning.

Gregg and Louise Kinsell, a married couple from Market Drayton, Shropshire, joined the Conservative party in the run-up to the election, motivated by a mix of patriotic pride, shared values and a liking for David Cameron. They signed up to join BattleBus2015 for its final stretch in the south-west, visiting four constituencies over four days: Stroud; Plymouth, Sutton and Devonport; St Ives and North Cornwall. The aim of the south-west tour was to turn the nine yellow seats of the Liberal Democrats into a sea of blue for the Conservatives – and the Tories won all but one.

The BattleBus operation is still being investigated, but the Kinsells firmly believe that, contrary to claims of Conservative party HQ, they and their fellow volunteers did promote local candidates. “The coach would pull in”, Louise says, “and they’d all be cheering. Honestly, we were like the big hitters coming down to make sure that we win. That’s exactly how it was.”

The couple recall that senior activists gave them scripts about the local candidates to memorise on the bus, in order to be ready to sing their virtues on the doorsteps of undecided voters. Specially prepared briefing notes helped them absorb local issues. And they claim they were handed bundles of locally focused leaflets and calling cards to slip through the letterboxes of prospective voters. The voting intentions of the people they called upon were carefully logged. The couple are clear that they were used as a tactic to “sway marginal seats”, and are angry at the ongoing claim of the Conservative party and some MPs that the BattleBus operation only promoted the national message. “If people are saying – and the MPs concerned in these areas are saying that it was part of a greater expense nationally for the Conservatives, that’s an obvious falsehood,” Gregg says.

Facebook Twitter Pinterest Nigel Farage, Al Murray and the winning Conservative candidate Craig Mackinlay at the count for the South Thanet seat, May 2015. Photograph: Matt Dunham/AP

But if there was one seat, among the 40/40 constituencies, that the Conservatives were most set upon winning, it was South Thanet in Kent. There, the Conservative party’s principal rival, Nigel Farage, would take on Craig Mackinlay in the most closely watched contest of the 2015 election.

Today the investigation into the Conservative victory in South Thanet is staffed by nine officers from the Kent police serious economic crime unit. The questions they are considering are familiar to those raised in the 2014 byelections. Were the hotel costs for visiting Conservative staffers in South Thanet – nearly £20,000 in total – properly declared?

After his election victory, Craig Mackinlay filed expenses of £14,838 for the short campaign – just £178 under the spending limit – but made no mention of the Royal Harbour hotel in Ramsgate where senior party workers had taken rooms. Was that an honest account of his expenses? And if not, who was responsible?

The search for answers has so far taken in boxes of internal Conservative documents, the testimony of campaigners, and a six-hour police interview earlier this month with Mackinlay. But a more basic question about the election remains disputed: who actually ran his South Thanet campaign? The list is longer than it should be.

At the top is the name Nathan Gray, Mackinlay’s election agent. In common with many of the “campaign managers” employed as part of the 40/40 strategy, Gray’s enthusiasm for politics was not matched by his experience. Then 26, he had never done the job before. (Gray denies any wrongdoing.)

In the aftermath of the great victory against Nigel Farage in South Thanet, Gray was largely written out of the story and replaced by Nick Timothy, a long-time special advisor to Theresa May who is now the prime minister’s joint chief of staff. In his book Why the Tories Won, Tim Ross describes how Timothy “was sent to take charge of the party’s flagging campaign to stop Farage in Thanet”. Grant Shapps even said recently that Timothy was “front and centre” in South Thanet. But he was not responsible for filing the expenses return and, when contacted about his involvement, a spokesperson stated that he provided “assistance for the Conservative party’s national team and would have given advice to any candidate who asked for it and indeed did so”. There is no suggestion that Timothy is at fault.

An analysis of the campaign written afterwards for the South Thanet Conservative Association credits someone else entirely: “In February [2015] CCHQ sent a professional team to help us. Their leader, Marion Little, is a very experienced election ‘trouble shooter’, and from the moment she arrived she effectively took control of the whole campaign.”

A Conservative staffer since 1984, Little had held the previous title “battleground director” of the Conservative party. And just as she had a formidable presence in the byelections of Newark, Clacton and Rochester and Strood, so she transformed the South Thanet Conservative’s constituency office into a military command post. Little was also not responsible for filing the election spending for South Thanet but she worked long into the night, battle planning and deploying troops: “Dear Team ‘South Thanet’,” she wrote in an email on 23 March. “Just to confirm that this weeks’ [sic] meeting schedule is as follows …” When Nick Timothy did make suggestions, they were run by Little: “Are we not putting ‘two horse race’ on everything?” he asked her in one email sent on 29 March 2015, before adding: “don’t we need to?”

Little didn’t respond when asked whether her role in South Thanet involved local campaigning.

Buses of activists also descended from London. Volunteers were dubbed the “South Thanet Soldiers”. One Labour campaigner, Peter Wallace, recalled seeing hordes of well-dressed young Conservatives working the constituency week after week. “They were like Terminators,” he said, “straight out of GQ, out of London and on our patch. They blew us away.”

Photographs and videos taken by Conservatives in the final weeks of campaigning show the scale of the resources used to bolster the party. There were visits from Boris Johnson and George Osborne, and groups of campaigners arriving on liveried Conservative coaches ready to work for Craig Mackinlay. On the morning of the election, party co-chairs Grant Shapps and Lord Feldman arrived with Mark Clarke and a coach of last-minute campaigners.

In the end Mackinlay defeated Farage in some style. The problem is that when Timothy and Little stayed down in South Thanet, they lived in some style too. The local spending limit in the election was just £15,016, but the bill for rooms housing the troubleshooters from CCHQ at the Royal Harbour hotel ran to £15,641 alone. Mackinlay denies any wrongdoing.

“They had a few rooms block-booked, yeah,” James Thomas, the owner of the Royal Harbour, told Channel 4 News. “All hotels become headquarters, unofficially sometimes,” he added. “Mr Farage was going to be defeated by them, so they made sure they had the right brains to do that.”

More hotel receipts, uncovered by Channel 4 News, showed more party workers staying at the Margate Premier Inn, some for 12 nights, with a total cost of £3,809. Little’s name was on the bill, but these costs were not declared in the local return or the party’s national expenses. It appeared to resemble the spending in the 2014 byelections – the money was off-the-books. The difference was that, this time, the Conservatives won.

The first report into the Conservative party’s election expenses was broadcast by Michael Crick on Channel 4 News in late January 2016. It was a short item on a slow news day, which simply asked why the cost of rooms at the Royal Harbour hotel in South Thanet had been declared as part of the Conservatives’ national – rather than local – campaign expenses. Why, Crick asked, would a team of top Conservatives be based at a small provincial hotel miles from anywhere if not to work on behalf of the Conservative candidate fighting Nigel Farage for the seat?

When investigative reporters at Channel Four News began to look at the threads connecting tactics in South Thanet to other high-profile Conservative campaigns, a tangle of receipts and emails revealed the party’s hidden spending elsewhere: undeclared hotels, busloads of activists on specialist missions, and senior CCHQ staff buried deep in provincial England.

For months, the Conservative party repeated that all their campaign spending was “in accordance with the law”. A member of the party’s governing body stepped in front of the cameras on 1 March to announce: “Channel 4 has got it wrong.” But eventually the Electoral Commission, which had been widely criticised as toothless, developed canines and sank them into the case. After pressing the party for three months, they were finally provided with seven boxes of papers in May 2016. The secrets they held would make police investigations inevitable but, even then, the Conservatives dug in.

One of the nation’s leading QCs was dispatched by Craig Mackinlay to Folkestone magistrates’ court to halt a Kent police investigation into election spending offences in South Thanet. He failed, and the detectives’ work continued. By the middle of June, 17 forces were conducting investigations into 27 sitting Conservative MPs. Since then, 12 police forces have passed files to the CPS to review, and up to 20 sitting MPs wait to discover if criminal charges will be brought, while other forces still sift through evidence.

In the meantime, the prime minister re-elected in 2015 has melted away, while the election expenses scandal continues to lap at the door of No 10 Downing Street. Theresa May’s chief of staff Nick Timothy and her political secretary Stephen Parkinson were both part of the team dispatched to South Thanet by CCHQ; both took rooms in the Royal Harbour hotel. Whether the reality of their work is reflected in the spending documents signed by Mackinlay is the essential question that Kent police must answer. The photograph in which May appears, walking with members of the senior campaign team on South Thanet’s seafront three weeks before election day, should also give the prime minister pause to consider her own party’s tactics.

Should the Conservative MPs still under investigation face trial and be convicted, May’s government will be imperilled. Her majority is just 17.

In deciding whether or not to prosecute, the CPS must consider two clear tests. The first concerns the public interest in pursuing prosecutions and is met easily: the integrity of our election process is at stake. The second test regards the chance of success at trial. This is harder to meet, because the law says that prosecutors would have to prove that the candidate or agent knowingly submitted a false return.

A likely defence is clear. In South Thanet, Mackinlay has told police that the senior Conservatives who came into his constituency to work on his campaign were not under his “direction or control”, so he is not accountable for their activity. Other MPs who enjoyed a visit from the BattleBus have said that they were told by party headquarters that it was a national scheme. While few of the MPs under investigation have publicly revealed what they knew of the real effect of BattleBus, some have stated publicly that they received an email from the RoadTrip founder and BattleBus organiser Mark Clarke, instructing them not to declare the costs. (Clarke declined our request for comment.)

After one year of investigation, the Electoral Commission has found categorically that at least some of the spending the party claimed was national spending was spent on “candidate campaigning” and therefore should have been declared by candidates on their local returns. They did not. This, the commission said, had potentially given them a “financial advantage over opponents”. It was the responsibility of the candidates and their agents to do so. According to the law, the responsibility for failing to do so lies only with the candidate and the agent.

It is too soon to say whether charges will be brought. Lancashire police recently told the BBC that it has dropped its investigation into one MP who received the BattleBus, David Morris. Press reports have cited police sources who suggest that prosecutors “might decide to make an example” of others.

But if prosecutors decide not to “make an example”, they may set a legal precedent instead. Future candidates will reasonably conclude that they can, with the assistance of their parties, circumvent the electoral laws intended to keep our democracy free and fair – and that parties and candidates alike may do so without facing any penalty.

The Channel 4 News investigations team are: editor, Job Rabkin; producer, Andy Lee; Channel 4 News managing editor, Ed Fraser