John McDonnell is sunk deep in a leather sofa in a corner of the bustling cafe bar of Norwich Playhouse, clutching a mug of tea. The shadow chancellor has taken a break from his regular holiday boating on the Norfolk Broads with his wife and a revolving cast of other family members to pop into the city for a chat.

It’s no secret that many of his colleagues view McDonnell as controlling, and see him and Jeremy Corbyn as fossilised relics of a politics they thought was consigned to the past. But the 65-year-old, a veteran of many picket lines who cut his teeth in his 20s working with Ken Livingstone in the lefty bastion of the Greater London Council, argues that the economic impact of the financial crash and its aftermath have become a fertile breeding ground for his brand of political radicalism.

“What the Labour party manifesto appealed to, I think, is people’s sense of insecurity,” he says. “They’ve had seven years of hard, hard austerity – their wages are worth less now than before the recession. The public services they rely upon have been cut to shreds, some of them are in absolute crisis. Even where they’re doing everything right, our young people are now clobbered with debt.”

The moment is ripe, believes McDonnell, for a fightback. “All those securities they’ve had have been stripped away. When you’re in the depths of a recession, that isn’t the time when people want to challenge the system, they’re too busy trying to survive. It’s when they’re told we’re coming out of a recession, growth is returning, and they’re not seeing the benefits of it, or they’re not seeing them quick enough.”

Restored by his break and from a visit by his five grandchildren, McDonnell appears relaxed, expansive and magnanimous. But when parliament resumes in the autumn, he says Labour must exploit Theresa May’s weakness ruthlessly – including by striking up alliances with disgruntled Tory backbenchers on individual issues.

“Individual backbenchers are coming back from their constituencies and confronting the same issues as the rest of us: people who can’t get a roof over their heads. There are food banks in Tory constituencies now; the pay isn’t rising enough to cope with inflation; there are high levels of household debt. They’re coming back thinking: ‘We’ve been at this seven years now and it hasn’t worked.’ We’ve got to divide them as much as we can; force the agenda about all those issues; and then use the parliamentary system to expose their position, and force as many votes as we possibly can.”

Senior Conservatives, liberated from the dominance of May’s advisers, Fiona Hill and Nick Timothy, are starting to think aloud about economic reform – and Philip Hammond is expected to use his autumn budget to provide some relief, including for public sector workers. But McDonnell believes it will be too little, too late. “A little bit of money isn’t going to work any more; the occasional sop isn’t going to work any more. I think they’ve boxed themselves in, because of their austerity programme.Never underestimate the Tories’ ability to cling on to office,” he says, “but they’ve put themselves into a position where a little bit here and a little bit there won’t work any more.”

McDonnell, a serial rebel who stood against Gordon Brown in 2007 to prevent him inheriting the premiership uncontested, and drew up an alternative budget every year Labour was in power, is regarded by many in his own party as more ruthless and Machiavellian than his longtime comrade-in-arms, Corbyn. Some believe he would rather stir up a revolution on the streets than win a peaceful victory at the ballot box.

The early months of the Corbyn project were often characterised by a bunker mentality, which appeared to be vindicated when they had to fight off mass frontbench resignations and a leadership challenge from Owen Smith. Just five months ago McDonnell described the leadership as being in a “360-degree struggle to survive” in the face of relentless attacks from the “New Labour establishment”.

McDonnell says he wants September’s party conference in Brighton to focus on ways of bringing the party’s near 600,000 members more closely into policymaking – though some see that agenda, too, as a way of tightening the left’s grip. “Rather than battles at Labour party conference over detailed constitutional amendments, it’s more important to get this year a real discussion about how this party becomes a social movement. How do we use that fantastic resource of those members?”

Yet even if clashes on the workings of the party’s internal machinery can be avoided, and despite the warm glow of the election campaign, Labour remains deeply riven over an issue McDonnell barely mentions for the first three-quarters of an hour today: Brexit. Days after the fresh crop of MPs took their seats in Westminster in June, 49 Labour MPs defied the whip to vote for an amendment to the Queen’s speech, tabled by backbenchers including Chuka Umunna, calling for the government to fight to retain membership of the customs union and the single market.

Earlier in the year, there was widespread soul-searching, and a series of high-profile resignations from the frontbench, when the leadership chose to back the government’s article 50 bill – and many MPs told their constituents they would fight for a soft Brexit.

McDonnell, like Corbyn, is a longtime Eurosceptic in the mould of Tony Benn, regarding the EU as a project geared more to the interests of financial elites than ordinary workers. During last year’s referendum, he and Corbyn backed the party’s decision to stay in but they rarely joined in with the enthusiastically pro-EU Labour campaign run by Alan Johnson, promoting instead a more sceptical message of “remain and reform”, alongside figures including former Greek finance minister Yanis Varoufakis.

Now, he urges his party to step back from what he calls “arcane” arguments about which EU institutions Britain will remain a member of in 2019 – and think instead about the practical benefits they want the new relationship with the EU27 to deliver. “The problem is, it’s become theological,” he says. “Whatever I say, I either get condemned as a hard Brexiteer, if I’ve stated an obvious fact – or I then get condemned for selling out the referendum. It’s like angels on the head of a pin. The bottom line for me, is the new relationship we have with Europe should be designed on the basis that we can implement our manifesto.”

On the free movement of people, for example, he would support any solution that would end the undercutting of wages: something he believes is possible without pulling up the drawbridge. “We’ve always defended freedom of movement in principle; but [it] cannot be on the basis that it undermines standards of living in this country – and therefore we address that issue in a practical way.” This could be done, he says, by measures to stop the exploitation of workers, better enforcement of the minimum wage and making sure public services are funded well enough to cope with new migrants to an area.

This consummate outsider, who has been advocating the same radical economic ideas from the sidelines of British political life for a generation, genuinely believes he could be about to win power. “We’re working on the basis that the government could collapse at any time,” he says. “We’ve got to do everything we possibly can to divide and demoralise them, and push that collapse, because that’s coming. But do that in a way that demonstrates that we are an alternative government, ready to go in.”

“People now understand what Jeremy Corbyn is as an individual. I think he’s won people’s respect. I think he’s grown in the job as well. People need to be absolutely clear about where we’re taking the country.”

To that end, he will be asking every shadow ministerial team to turn their own section of the manifesto into a detailed “manual for government” – even drawing up draft legislation to implement key policies. “It was the manifesto for an election. What we do now is we take each of those policies, we turn them into a working document which is a manual for implementation. You draft that manual in detail, you draft legislation and have it on the shelf ready to go.”

Repeatedly describing himself as a “bureaucrat”, he says his own team have been out and about in the City, offering reassurance that their policies would not destabilise the economy – and obviate the risk that nervous investors would react with panic to a Labour victory at the polls.

“The issue for us is to stabilise the markets before we get into government, so there are no short-term shocks,” he says. “We’re sitting down with people in the City – asset managers, fund managers and others. I’ve been to the London Stock Exchange. I’ve said: ‘Look, if I believed half the stuff in the Daily Mail about myself, not only would I not vote for myself but I’d be terrified as well. But let me reassure you, this is what our plan is.”

He says he will be asking veterans of previous Labour governments to offer their advice to what is still a relatively young, inexperienced shadow cabinet of Corbyn loyalists, stripped of some of its more experienced heads by the walkouts of last summer. (He makes no suggestion, however, that any of those with ministerial experience will be invited back into the fold.)

He talks at length about policy – on social care, a hint on heavier property taxes and tackling the housing crisis. Some serious thinking is going on in the party over the summer about how to capitalise on Labour’s popular manifesto. And it is clear that McDonnell, with his sights on entering government, is integral to that project.

Later in our chat, a cafe customer comes over to shake McDonnell’s hand. She introduces herself as Ruth, a lapsed Labour member whose faith in the party has been restored by Corbyn’s leadership. McDonnell urges her to rejoin. Perhaps an even more unlikely political idol than Corbyn himself, McDonnell is clearly enjoying this unexpected fandom, late in life.

“We gave people a bit of hope,” he says. “We won’t hold on to that feeling of hope unless we can go back to them and say, ‘That hope has a secure foundation’.”

But he believes the Tories’ precarious position means Ruth won’t have long to wait. “Their time is over. They’ve had seven years to turn this economy around, seven years to turn this country around and they’ve failed. The reason they’re frightened of us is because we’ve exposed them. We’ve exposed them as incompetent failures. We’ve exposed them for what they are.”