We interrupt John McDonnell in the middle of preparing his speech for the Labour conference. When he performs in Liverpool on Monday, is he planning to entertain his home city with some of his jokes? “I’m trying to avoid funny bits,” he grimaces. “They get me into trouble.”

One of his controversial cracks has been proved true. Back in the early summer, he told a rally of supporters that Labour MPs attempting the removal of Jeremy Corbyn were “fucking useless” as plotters. With McDonnell reprising his role as campaign manager, Corbyn has been re-crowned with an even bigger vote than he received the first time around a year ago.

There are many Labour colleagues who regard the shadow chancellor as the real brain of the operation. While Corbyn is the frontman, it is McDonnell who is the mastermind of their takeover of the party. “Rubbish!” he exclaims. “We have worked together for 30 years, you know. We have different skills, different talents. We complement one another. Simple as that.” He also wants to kill the persistent suggestion that Corbyn will stand down at some point and it is McDonnell’s ambition to succeed him. “I will never stand for Labour leader,” he says. “Never.”

McDonnell is commonly portrayed as the more aggressive of the two, but today he is seeking to pour emollience on the divisions that have riven the party. The leadership challenge “distracted us for a couple of months, but I think we can pull it together”. McDonnell insists that the vitriol exchanged over the summer can be put behind them. “In terms of what’s been said during the campaign, what is said on tour, remains on tour.”

Facebook Twitter Pinterest Jeremy Corbyn after being announced as the winner of the party’s leadership contest. Photograph: Oli Scarff/AFP/Getty Images

The challenge was triggered when 172 Labour MPs declared no confidence in the leader and more than 60 resigned from the frontbench. How can that be unsaid or undone? “The response to that is, tell us what the problems are, and help us address them. The offer to each of them is to say: what is your criticism? Is it around presentational style, is it around a policy area you are worried about, is it around a professional relationship, or an organisational issue? What is it?” He argues that Labour is actually a lot more united than it seems. “Policywise, well you have seen it in this leadership election. The most common phrase used by Owen Smith [the challenger] is, ‘I agree with Jeremy.’ Apart from Trident and Syria – and I always argued for free votes on those because I knew they were conscience issues – on virtually every other area there was consensus.”

He urges the MPs who quit the frontbench to unresign. “We get them back, hopefully. Quite a few have been contacting us.” How many? “I won’t go into numbers, but a few. We just want to get them back to work. I think they will accept the mandate.” He says that a welcome mat would even be laid out for the defeated challenger should Smith have a change of heart about returning to the shadow cabinet. “Of course. He is extremely bright, he wants to get things done, he was a doing a great job and he was a great performer in the chamber.”

Facebook Twitter Pinterest McDonnell says even Owen Smith would be welcomed back into the shadow cabinet. Photograph: Robert Perry/EPA

Many Labour MPs say that they would be more likely to return to the frontbench team if the parliamentary party were to regain the right to elect most of the shadow cabinet. “There is an open discussion going on,” he responds cautiously. “There will be a wider discussion within the party at every level. But you need time to do that.” He floats the suggestion of “a special rules conference” to introduce changes that would mean “more member involvement”.

One consequence of the leadership contest has been to swell Labour’s membership to the point where it is now the largest party in western Europe. “We’ve got 680,000 members now. We could be in a situation by next year where we have a million members. We have got to come to terms with how you organise that and how to make sure those people are fully involved.”

The membership surge and an accompanying increase in donations has also made the party considerably richer. But the shadow chancellor can’t say by precisely how much because, according to him, the party’s general secretary won’t tell him. “I don’t know. Iain McNicol never showed me the bank account.” Shouldn’t he? “Tell him that, will you? Tell him that. Show us the money.”

During the campaign, there was an acrimonious row with McNicol when McDonnell alleged that officials at party headquarters were conducting a “rigged purge” of Corbyn supporters. He denies that a revenge counter-purge of the general secretary and his staff is now planned. We ask him directly: does he want to get rid of McNicol? “No, no,” he insists. “Happy for him to remain.” It’s not the warmest of endorsements.

He also denies that there is a plan to punish Labour MPs who refuse to fall into line: the decapitation-by-deselection that is undoubtedly feared in the parliamentary party. “We are not in favour of mandatory reselection. Every assurance has been given to them. Jeremy and I, on several occasions, almost ad nauseam, have been saying to party members exactly the same. If the boundary stuff goes through, there will be reselections, but that is not a threat. We’ve said the existing rules will apply. I don’t know what more we can do.”

It is one of McDonnell’s more attractive qualities that he readily acknowledges that both he and Corbyn need to learn from a “really tough” 12 months. “It’s been hard and hard lessons have been learned.” What lessons? “Presentation, how we present our arguments,” he offers as an example of where they need to up their game. Corbyn’s victory speech was a notably slicker performance than the one he delivered at his first coronation.

McDonnell also suggests he doesn’t entirely disagree with Labour MPs who complain that the leadership team has been dysfunctional. “I always thought I was a political artisan and not a bad bureaucrat because that is my background,” he says. But he accepts they need to work on “how we operate. I am trying to pick people’s brains all the time.”

The party’s MPs often speak of themselves as people under siege. It is not so commonly acknowledged that the leader and his shadow chancellor also feel encircled by hostile forces: the parliamentary party and the media especially. “The media scrutiny has just been extraordinary,” he says. “I think even elements within the media itself have been surprised by the vehemence of it.”

His conference speech will warn Labour to be prepared for an early election. Though Theresa May has ruled out trying to cash in her poll advantage, McDonnell thinks she might change her mind. “You never trust a Tory. So it could come at any time. She could always dress something up, couldn’t she?”

They need to think about a “first 100 days” project to detail what a Corbyn government would do immediately it took power. “What happens if there is an election tomorrow? Have you got your first 100 days in place? I want to start that off.”

Corbyn leadership win shows Labour is now a changed party Read more

His central responsibility as shadow chancellor is to convince enough voters to trust Labour with the economy, a crucial area in which it failed at the last election and, according to the current polls, is doing even more badly now. He has lost one of his handpicked advisers, Professor David Blanchflower, who quit the shadow chancellor’s economic advisory committee complaining that McDonnell did not have a credible plan and lamenting that “a Labour party led by Jeremy Corbyn has no chance of winning a general election now, in 2020, or at any other time”. McDonnell shrugs: “I didn’t understand it. I really didn’t, because I had been getting on all right with him.”

He argues that Labour has been proved right about austerity, as evidenced by May’s decision to ditch George Osborne’s fiscal plan. “It is interesting they are coming on to our agenda,” he says. “Technically we win the argument all the time. We don’t win the narrative and narrative beats technical. We haven’t had a clear and consistent narrative so we have got to go back almost to square one.”

With this emphasis on the importance of “narrative” in politics, he almost sounds like Peter Mandelson. Labour needs to make its case for more investment and redistribution of wealth in language “people can understand, and that is our challenge in these coming months”.

The other is to try to bind together a bitterly divided party. McDonnell is still fuming that some Corbyn-supporting members were barred from voting, often on the grounds they had used a word on a banned list drawn up by party officials. One of those words is “traitor”. It prompts one of his jokes. “When we sing the Red Flag this time, we are going to have to delete the word traitor, aren’t we?”