The Conservatives are under fire after the Electoral Commission fined them a record £70,000 for a string of breaches of election rules.

The Tories failed to report expenses properly for the 2015 General Election and three by-elections the year before.

The watchdog issued its report after a long-running Channel 4 News investigation into Conservative election spending, including the question of whether the party incorrectly recorded money spent on its “battle bus” tour.

Party activists were bussed to key marginal seats, and some money that should have been declared as spending on campaigns in individual constituencies was wrongly declared as national spending.

The Electoral Commission said that meant there was “a realistic prospect that this enabled its candidates to gain a financial advantage over opponents”.

It also said party workers sent to South Thanet to try to hold a Tory seat under attack from Ukip promoted and supported the local candidate, but their work was not declared as local spending.

Today, senior Tories attempted to play down the scandal – but they made a series of questionable claims in the process.

“We have complied fully with the Electoral Commission throughout their investigation.” Theresa May

This is directly contradicted by the Electoral Commission’s version of events. The watchdog said in its report:

Specifically, the commission said the Conservatives failed to hand over material when requested, and only did so fully after the watchdog lodged an application with the High Court for a disclosure order.

Refusing to hand over documents, then only agreeing to do so under the threat of being dragged to the High Court, isn’t “complying fully”.

In fact, “lack of co-operation by the Party during the investigation” was one of the reasons the Commission imposed such a large fine.

“We’re talking about something like half a percent of the total spending of the national party.” Sir Oliver Letwin

Former Conservative cabinet minister Sir Oliver Letwin defended the Conservatives in a BBC Radio 4 Today programme interview this morning.

It’s not absolutely clear what calculation he’s making here. If we only counted the £104,765 which the Tories missed off their return, that would indeed by just over 0.5 per cent of the £15.5m they spent on the 2015 General Election.

We could quibble with this, and ask why Sir Oliver doesn’t the include the £118,124 which was either not reported or incorrectly reported.

This would come to around 1.5 per cent of the party’s 2015 spending.

But more importantly, this is arguably not a useful way of putting the spending breaches in context.

While the spending limits for a party’s national campaign are relatively large, the maximum an individual candidate can spend on his or her local campaign is much smaller.

In South Thanet – the key seat investigated by the Electoral Commission – Conservative candidate Craig Mackinlay was only allowed to spend £15,016.

With such tight limits imposed on candidates, a relative small amount of extra money or resources spent locally could give one party a considerable advantage over another.

“My suspicion here is that actually, there isn’t any guilty party or somebody who was trying to do something wrong… I think what we’ve seen is people making a whole series of human errors.” Sir Oliver Letwin

Sir Oliver’s repeatedly suggested that the breaches can all be explained by mistakes rather than criminal conduct – but this is a matter for the Crown Prosecution Service to decide.

The Electoral Commission has referred Simon Day, the Conservative Treasurer at the time (and a former Chief Executive) to the police “for consideration as to whether to investigate him for a potential offence” under the Political Parties, Elections and Referendums Act.

It is now a matter for Scotland Yard to decide how to proceed. The watchdog said an offence would be committed if a treasurer “knowingly or recklessly made a false declaration”.

The phrase “human error”, which Sir Oliver used in the interview, was specifically rejected as a defence by the commission, who said it “did not constitute a reasonable excuse”.

Separate to that police matter, 12 forces have sent files to the Crown Prosecution Service as part of a parallel investigation into returns submitted by individual candidates.

It will be for CPS lawyers to decide whether to pursue prosecutions.