With the government in choppy waters and the economy heading for Brexit turbulence, John McDonnell has been practising his captaincy skills. To relax away from Westminster and clear the mind, the would-be chancellor of the exchequer reveals that he and his wife, Cynthia, have been heading for the Norfolk Broads for sailing breaks.

“We decided we needed to do a sport together, some activity, so we took up dinghy sailing.” He hoots with laughter, recalling the original name of the vessel they bought a few years ago. “Believe it or not, I think it was called the Morning Star. Pure coincidence! The name has dropped off now. These days we just call it the boat.”

He scrolls through some pictures on his phone until he finds one featuring his friend Jeremy Corbyn, who stayed with the McDonnells recently at a hut they own next to their mooring.

“Look – there he is. Classic Jeremy,” he says pointing at a figure in the distance, standing perilously close to what look likes the end of a jetty. “Can you see him? And I’m saying ‘get down! You are going to cause a byelection!’” McDonnell is under no illusions about his or his wife’s sailing abilities. “We’re rubbish,” he admits. “People get off the water when they see us coming because (he shakes his head) … it’s terrible.”

The shadow chancellor, a self-proclaimed Marxist, can be engaging company. He has rarely been asked much about this side of his life, the one away from politics – but he seems comfortable opening up about it. “I read a lot, theatre, cinema, the rest of it,” he says. He is happy, too, talking about how he got to where he is today, the early steps on his extraordinary journey to the top echelons of the Labour party.

McDonnell was born in Liverpool, did his A-levels in Burnley, then made his way via Brunel University into the trades union movement while at the same time working as a “house father” in a children’s home in Hillingdon, west London, for 13 years, looking after 10 youngsters. These days he has his own large family: two daughters from a first marriage, a son from his second, and five grandchildren. “They’re all descending on me this weekend, the northern tribe,” he says. “It will be fun.”

Now aged 66, his time to enjoy life outside Westminister is more squeezed than ever. Relatively late in life, huge levels of political responsibility have been thrust on his shoulders. Throughout the New Labour years McDonnell was a lonely figure on the party’s far left, along with Corbyn. Both were largely ignored by the media. Before Wednesday’s budget, however, he is being scrutinised as never before – not only because he is in charge of Labour’s economic policy, but because he is in effect auditioning for the chancellor’s role.

If he feels pressure, he disguises it well, joking about his years opposing his own party. “Gordon Brown, bless him, said the other night that John McDonnell was always shadow chancellor because he was always the shadow. I’d always do an alternative budget [to Brown’s] and get up and present it in parliament.”

These days, though, McDonnell is having to think not about being the rebel within, but about leading and governing. He says that Corbyn approached Theresa May’s office last week to ask if all the shadow ministerial teams could have access to civil servants, so they could learn the ropes of running government departments.

What could happen is beginning to sink in, he says. Recently the shadow cabinet devoted an entire meeting to hearing from the former head of the civil service, Lord Kerslake, and others with experience in Whitehall, about what government actually involves.

“A whole team came in to talk about structures of government, how government operates, style of government, culture of government, that sort of thing.” McDonnell says he is seeking advice wherever he can find it, including from Brown. “I’ll be sitting down with Gordon and some of his people that he had around him as well. It’s exciting. The prospects are exciting, but I am not getting carried away.”

McDonnell is cautious. He has not spent a lifetime fighting Tories without knowing their ability to reinvent themselves to keep power. Of May’s current government he says: “I think they’re disintegrating, on a slow retreat to oblivion, but I never underestimate them and their ability to cling on.”

Given that he is from the far left, he also knows that he has to bring his own party and more of the electorate with him. His own economic policy programme involving the mass renationalisation of public utilties, £250bn of public spending, higher taxes for the better off, big cash injections into the public services including an extra £6bn immediately for the NHS, and a huge housebuilding programme, is more leftwing than any put forward by Labour in living memory.

It scares the living daylights out of the remnants of New Labour. As does McDonnell himself. He is seen by many MPs on the right of the party and those who would place themselves in its political centre as a dangerous revolutionary (more so than Corbyn) – and not just on economics. Many are convinced his real mission is to ensure total control by the hard left of all Labour party structures, from the ruling NEC down to local constituency parties, so he and Corbyn can show their true colours and squash opposition once in power.

If that is the agenda, he is hiding it for now, focusing on messages around which much of the party can unite relatively easily. Last week he set out five demands for Philip Hammond’s budget that caused no serious disquiet among Labour MPs: to pause and fix universal credit; provide new funding to lift the public sector pay cap; provide large-scale extra funding for infrastructure; properly fund health, education and local government; and launch a large-scale public housebuilding programme.

With another general election a distinct possibility soon, McDonnell and Corbyn are packaging themselves not as the radical old left reborn, but as the new left in the mainstream. McDonnell says the country at large is coming to realise that the Tories govern only for the elites and that his agenda answers contemporary concerns about that unfairness.

“This corporate capture by this government isn’t just alienating the ordinary working-class people because of the impact of austerity, it’s actually undermining the base of those middle earners, if you like,” he says. “When people are having those school gate conversations saying, ‘oh they’ve just laid off so and so, our teaching assistant in the class’, or ‘last night I was at A&E and we waited eight hours before we actually saw anyone’ and then they read about the Paradise Papers and the super-rich avoiding taxes, no wonder people are getting angry.”

Why though, given all this and the hopelessness of the Tories, is Labour not soaring ahead of the Conservatives? And why are he and Corbyn still lagging behind May and Hammond in the economic credibility ratings? In today’s Opinium survey for the Observer, Labour is just two points ahead of the Tories (42% against 40%) after one of the most catastrophic periods for a Conservative government in living memory.

Is it not extraordinary that the prime minister and chancellor remain the most trusted on the economy, with 36% preferring them to run the economy, against the 28% who favour Corbyn and McDonnell?

McDonnell insists the ratings will improve. He was proved right before the last election when no one thought Labour would surge. “No one believed me,” he says.

The shadow chancellor has been looking back at the records and says sudden leaps only happen for oppositions as a result of cataclysmic events. “The Labour party went ahead in the polls immediately after [the UK exit from] the ERM. That is when the 20 points emerged. So, in opposition, unless there’s something cataclysmic, you don’t usually don’t have those leaps, but you steadily build and that is what we are doing.”

Given how far he has come in the last three years, McDonnell prefers to think the dream revival of his career will continue. He and his great friend Corbyn have enjoyed a staggering journey from obscurity to the brink of power since the election of 2015.

But surely he must worry that it could go wrong. He must know fairytales aren’t real, that chancellors and prime ministers fall out.

If they become neighbours in Downing Street might not the relationship between Corbyn and McDonnell become as acrimonious as that between Blair and Brown?

“Jeremy and I have worked together for 35 years,” he says. “We’ve watched our backs for 35 years. Sometimes we’ve been swimming against the stream on our own. We’re mates, we’re friends, our politics are the same. We work very closely together.”

Wednesday’s budget will be the next staging post, McDonnell believes. He thinks there will be general election soon because there is nothing Hammond or May can really do this week or in the near future to revive their party.

“If Hammond uses the budget to try to buy off the individual factions in the Tory party it’ll be chaos,” says McDonnell. “And if he doesn’t, they will remain just as split as they are now.

“They’ve got no mission, no objectives and I think they realise. What is the point of them being there?”

• This article was amended on 20 November 2017 to correct to £250bn the total of John McDonnell’s plans for public spending. It originally mistakenly gave the total as £500bn.

