Tom Watson’s friends say he is half the man he used to be. He has lost eight stone, and over the past couple of years a fitter, sleeker, more energised Watson has emerged; he is now retraining as a gym instructor. Those of a less friendly disposition would also say he is half the man he used to be. They accuse Labour’s former deputy leader of betraying his party, by undermining Jeremy Corbyn time and again. Longer-term critics would say he was never much of a man, and wish they could have dispensed with him years ago.

It’s 11am on Friday 13 December, the day after the night before, and the inquest into Labour’s trouncing has already begun. Watson is upset, emotional and exhausted. He has been up all night, and is about to phone friends who have lost their seats to offer support. There are lots of them. Even West Bromwich East, the seat he held for 18 years, has gone Tory – for the first time.

Watson talks about the reasons for the defeat: a familiar list including Brexit, Corbyn, a manifesto that overpromised, antisemitism, a failure to unite the left and centre of the party. “I don’t even know what the message of our campaign was,” he says. “There were announcements everywhere, but none of them got through because there were so many. You knew what Boris Johnson was: get Brexit done. What was the Labour strapline?” Has politics got to be so simplistic? “No, politics is very complicated. But you have to be able to condense what you stand for into a mantra or a single message.”

As well as the lack of a clear message, Watson thinks Labour was all over the place organisationally. “I don’t actually know where the key-seat list was for this election. I don’t know whether rigorous, disciplined assessment of which seats were under threat and which could be won took place. In 2010, under Gordon Brown, we were very disciplined and many seats were saved from defeat as a result. The mechanics of elections really do matter.”

There is, he says, a simple question that the party must address. “Does the Labour party in its current form actually want power? The ultimate betrayal of working-class people is not to take power when you can, and if you are a party that believes in power through elections, then that requires pragmatism, prioritisation, compromise and collaboration.”

Watson with Jeremy Corbyn at the Labour party conference in 2016. Photograph: Getty Images

You sense Watson could talk for hours, possibly days. This is therapy, a process of understanding – and part of that process is working out what part he played in all this himself. Did his party get hammered because Corbyn and his advisers refused to listen to Watson’s call for a broader church? Or was Watson part of the reason for the hammering?

Some have argued that Labour lost because remainers such as Watson forced them into a fuddled compromise on Brexit. (“By all means criticise me for the position I took, but at least it was a position,” Watson says. “Something I strongly believe in, to the point that it influenced my decision to leave elected office, because I would have found it very difficult to hold the party line and I didn’t want to dissemble.”) Others have suggested that Watson failed them by standing down as deputy leader and an MP.

It came as a shock, because politics had been his life, as a trade union officer and member of parliament. But Watson had decided he’d had enough; that politics wasn’t good for his health. “I did worry that the brutality and hostility would get to me in the end.” Can he give an example? “Well, two weeks before I resigned, a guy was arrested for giving me a death threat. He was a Labour supporter. The police got in touch and said, ‘We’ve arrested this guy’, assuming I knew about it. But I didn’t. The Labour party had sent out a fundraising email that he had responded to with a death threat. The party reported it to the police, but didn’t tell me.” Why not? “I have no idea – it’s neither here nor there now. The point is that the brutality and hostility is real and it’s day to day. So I just thought: now’s the time to take a leap, do something different – you’ve had a good innings. You’ve done good stuff. Go now. And it was quite quick.”

I was asked to order an Ikea sofa for Peter Mandelson’s office. I didn’t know I’d be doing that when I joined at 15

So the brutality and hostility was from outside the parliamentary party? He shakes his head. No, he says, there was plenty from inside. He reels off a long list. “The return of political factionalism. The idea that you could be portrayed as the enemy within. People questioning your motives. Social media pile-ons. Condemnation from powerful trade union general secretaries on broadcast media. Internal bickering. On their own, you deal with them and they’re a normal part of life. Combine them, and you’re carrying a very heavy load. And sometimes you’ve got to realise when that balance of life shifts and there are other things that are more rewarding.”

Now, Watson is publishing a memoir, aptly titled Downsizing. He assures me his new life is going to be smaller in every way. He will dedicate himself to his two children Malachy, 14, and Saoirse, 11, staying fit, and helping others get fit, too.

A few years ago he weighed 22 stone and was diagnosed with type 2 diabetes. “I had overwhelming thoughts of death. I thought: you can’t walk upstairs without feeling as if you need an oxygen mask. How have you got to this point?” Watson realised that he could no longer fully function as a father, politician or human being. “You get to that point where you go: I’ve got kids – I want to stay alive. You think what a useless dad you are: you can’t kick a ball around with Malachy, you can’t jump in the pool without emptying it.” Watson went on a fat-heavy, low-carb, sugar-free diet. He ate bacon and eggs for breakfast, full-fat cheese and walnuts for dessert, and glugged back glasses of double cream if he still wasn’t sated. And the weight just fell away. Even more astonishingly, his diabetes went into remission.

***

A couple of weeks before the election, I meet Watson in Bewdley, a small riverside town in Worcestershire. We are three miles from Kidderminster, where he grew up, and barely a mile from where he lives today. Watson recently moved in with his new partner, and shares the parenting of his two children with his wife, Siobhan, from whom he separated in 2010, and who lives in Yorkshire. He wanted to meet here so we could go for a walk, and so he could show off his renewed stamina and love of the natural world. Most days Watson walks around 12km, and he looks a good decade younger than he did. For many years he could only wear outsized suits. Now, aged 52, he can wear what he wants, and he’s loving it. Even his glasses look lighter.

With his ex-wife, Siobhan, during the election count in his constituency of West Bromwich East in 2005. Photograph: News Team International

His political life goes back generations. His grandmother was a communist until after the collapse of the Soviet Union, and his mother and father were both Labour; she was a secretary, he was an odd-job man, before both retrained as social workers. He remembers them running a tiny committee room from the family home in Kidderminster in 1974 – the intense seriousness of it all, the boisterous debates in the living room. He found it intoxicating.

By the time Watson was 12, he was filling envelopes for the 1979 election from the more substantial committee room of the Power Loom Carpet Weavers’ and Textiles Workers’ Union. At 17, he was hired as a £5,400-a-year trainee librarian at the Labour party’s Walworth Road HQ, on the strength of being the only candidate who had seen the job advert, and because he knew how to use a word processor. He went to Hull University as a mature student in 1990, was elected president of the student union in 1991, became the Labour party’s youth officer in 1993, and worked on the 1997 general election campaign, before leaving to become national political officer of the Amalgamated Engineering and Electrical Union. Watson was elected MP for West Bromwich East in 2001.

He was 17 stone when he entered parliament, with a reputation as a bruiser – partly because of his appearance, partly because of his trade union history, and partly because he could deliver a devastating attack with the understated calm of a mafioso. To his credit, Watson has rarely chosen easy targets, instead going for global media magnates and leaders of his own party. The one time he did was as campaign manager for Liam Byrne in the 2004 Birmingham Hodge Hill byelection. Watson took responsibility for a filthy campaign including a leaflet that featured the line: “Labour is on your side; the Lib Dems are on the side of failed asylum-seekers.” He says he “winces” when he thinks about it now. “I was so hungry for winning, I just took my eye off the ball. We won it by a few hundred votes, but I’m very sorry about it.”

There are people around Jeremy who live in a binary world of left and right, good and evil, us and them, in and out

For all Watson’s scheming, he was clubbable. Indeed, he says, this is why he put on even more weight. “I loved eating, I loved the craic, I loved people’s company, I loved long lunches with left-leaning journalists and a few beers after work.” He would often start the day with two or three bacon butties in the members’ tea room. Watson says as soon as he entered parliament he became more class conscious, aware that some regarded him as an oik. “I’d never order three bacon butties at the same time because that was a social faux pas even I was aware of.” What stopped him – that it made him look greedy or plebby? “Definitely plebby.”

It’s funny, he says, how desperate he was to escape Kidderminster as a young man, only to spend decades being condescended to in the Commons. “There’s a terrible snobbery. It was a very exciting time, but I was patronised by people who were better educated and very ambitious in a classic way.” He mentions a stint working as deputy general election coordinator in 1997. “Benjamin Wegg-Prosser, who worked for Peter Mandelson, brought the Ikea catalogue to my desk and asked me to order the right sort of sofa for Peter in Millbank Tower, for when the great lord arrived to direct the campaign. I sat there thinking: when I joined the Labour party at 15, I didn’t know I would be buying sofas for Peter Mandelson.” He pauses, before adding how well he gets on with Mandelson these days, and what a charming man Wegg-Prosser is.

By 2006, Watson had been promoted to under-secretary of state at the Ministry of Defence, successfully campaigning to ensure that soldiers shot for cowardice in the first world war received a posthumous pardon – one of his proudest achievements. It looked as if he was destined for swift elevation to a senior position. But then came the first of his famous acts of disloyalty – and the first in a series of resignations. After Tony Blair announced he would not stand down as prime minister, Watson was a signatory on a letter saying the leader should go, after nine years. The government’s chief whip, Jacqui Smith, told him he had to withdraw his name. The next day he resigned instead. Blair responded that Watson’s intervention was “disloyal, discourteous and wrong”.

Today, he says this is one of his great regrets. “It was rash and chaotic. I’ve never said this publicly, but I shouldn’t have done it that way.” Why not? “If I’d lost confidence in him, I should have told him to his face, and done it more respectfully. I was a kid, a junior minister, in a government that had achieved great things, and he deserved better.” He’s being candid, but knowingly so: it’s not the only time Watson will say he’s never said something publicly before, as if telling me where the story is.

Watson returned to government under Gordon Brown a year later, first as a whip, then as minister for digital engagement and civil service issues. He had lost none of his love of the craic; there was nothing he liked more than a night out on the Guinness. How many could he get through? “I could do eight or nine Guinnesses on a Friday.” Is he being conservative? “I could have probably got to a dozen. Not beyond that, because I’m not a great drinker. But I liked drinking a lot.” Did he worry that he was an alcoholic? “No. I never felt addicted. I could always not drink.”

‘The Labour party only wins elections when it’s the broadest of churches,’ said Watson the day after the defeat. Photograph: Ben Quinton/The Guardian

Watson has resigned under every Labour leader he has worked for (from Brown’s cabinet for personal reasons; and as the Labour party’s campaign coordinator under Ed Miliband, after a row about candidate selection and losing faith in his boss). Watson insists it’s a record he’s not proud of, but I’m not so sure. He stresses that he’s never been sacked. In 2009, he joined the culture, media and sport select committee, where he found himself at the heart of an investigation into the phone hacking scandal. “It became my expert subject. It was overwhelming and obsessional. I was very boring to be with in those days. I couldn’t think of anything else. I was utterly driven by it.” He made headlines with his forensic interrogation of Rupert and James Murdoch. In 2011, the News of the World was closed down after detectives said as many as 4,000 people had been hacked.

A year later, Watson began another high-profile campaign to expose a so-called “VIP paedophile ring”. The campaign will probably be remembered for the attention he drew to “Nick”, a man who accused senior politicians and establishment figures of sadistic sexual abuse and murder, prompting a £2m investigation by the Metropolitan police. Nick was eventually named as Carl Beech, who in July this year was sentenced to 18 years’ imprisonment after making false allegations against Conservative politicians Harvey Proctor, Leon Brittan and former prime minister Edward Heath, among others.

How did Watson feel when Beech was found guilty? He pauses. The seconds pass slowly. “I felt sick as a dog for the genuine victims, that this one case and this one person had set it back. There are people even now who are the victims of child sexual abuse who are not going to bring their cases to the attention of the criminal justice system.”

Did he feel that at any point he mishandled the situation? “Yes, when Leon Brittan died.” After Brittan’s death, Watson wrote an article in which he repeated a claim by Beech that Brittan was “as close to evil as any human being could get”. “I shouldn’t have done that and I regret that, because it was an emotional response. I’ve apologised for that. It was a mistake. I don’t want to beat myself up for every mistake.”

While undoubtedly well intentioned, Watson’s pursuit of a Westminster paedophile ring felt like an act of hubris; he seemed to have developed a sense of his own invincibility. Lady Brittan called for Watson to resign or be sacked, but he still insists he did more good than harm. “Please look at the lifelong predatory child sex abusers who went to jail as a result of the evidence provided to the police from whistleblowers who came to me. Please look at the work of the independent inquiry into child sexual abuse, which came about as a result of working with a group of cross-party MPs. Look at the work we did on bringing a group of MPs together to look at institutional abuse in music schools.”

***

We head off for a stroll along the River Severn. The four miles from Bewdley to Upper Arley is one of Watson’s regular routes, and he is relaxed, easy company. As he walks, Watson chats about his love of music and the joys of nature – the cormorant that flies up and down the river, the cobalt blue of the kingfisher flashing past, the pike that always breaks his fishing line. Meanwhile, I’m telling him how I have spent much of the past four years screaming at Corbyn’s Labour. He nods sympathetically. No, I say, don’t get me wrong – I’ve been screaming at you as well. Why, he asks. Because you were a supremely disloyal deputy. Because at times you seemed to be revelling in the chaos, making political capital out of sectarianism, antisemitism and Brexit division.

Does Watson believe he was an honourable deputy? ‘Oh God! Well, I hope so.’ Photograph: Ben Quinton/The Guardian

He counters by asking if I would have preferred him to stay quiet and betray his principles. “I was trying to stand up for pluralism. The Labour party only wins elections when it’s the broadest of churches. But there are some sections around Jeremy and the Unite group that live in a binary world of left and right, good and evil, us and them, in and out. You can’t run a two-party system without nuance and complexity, pluralism and respect for difference. Because ultimately you end up becoming a sect not a party.”

Corbyn and Watson, both lifelong rebels (Watson from the centre ground, Corbyn from the left), might have been the dream team, uniting two factions. I remind Watson that when he was elected deputy in 2015, he said: “My role is to unify the party, hold things together and make it work in difficult times.” He smiles. “Well, what’s that Meat Loaf track? Two out of three ain’t bad? I’ve definitely not unified the party, have I? Nobody has done that. But I have helped hold it together in more ways than some people appreciate. And I’ve definitely tried to make it more functional.”

He says he sensed the partnership with Corbyn was doomed from day one. “We had just won the leader and deputy leader ballots, and we were in this room on our own, and the first thing he said to me was, ‘We’ve got our party back.’” For Corbyn, he had wrestled control from the Blairites, but Watson had never felt part of any faction. “I remember thinking to myself, I’ve never really lost this party. We’re going to have a bit of fun here, Jeremy.”

He talks about how ineptly his party dealt with antisemitism, and the Equality and Human Rights Commission (EHRC) inquiry. “I made private representations on what we needed to do to stamp it out, and I still think that we need to do a lot more work. If the EHRC inquiry goes the wrong way, a great shame will befall the institution that I love. One of the reasons I joined the Labour party in the 1980s was because I was an avowed anti-racist, and you can’t be discriminatory in that view.”

But he is fulsome in his praise of Corbyn as a man: his sincerity and devotion, the kindness he showed Watson’s children. He is less kindly disposed towards Corbyn’s advisers, and is furious that they involved themselves in the party’s investigation into antisemitism. “Why were Andrew Murray and Seumas Milne anywhere near what are internal disciplinary racist cases? Milne is the press guy, Murray is the trade unionist seconded to help do whatever he was doing in the leader’s office. They should not be involved in that process, period.” He pauses, before returning to the subject of Murray. “I did find it surprising that he would be in the Labour party, given that he is an avowed Stalinist.”

Watson briefly lived in a houseshare with another Labour power broker, Unite general secretary Len McCluskey, after he separated from his wife in 2010. McCluskey and Watson had an acrimonious falling out. Watson says he can tell me exactly when: it was July 2016, the Saturday of the Unite conference, after Corbyn lost a vote of no confidence by Labour MPs, a seemingly disastrous 172-40 against.

“The last time I spoke to Len was when we were having private talks about how we handle the vote of no confidence in Jeremy. He was basically offering a fairly long-term departure date for Jeremy to go – hints at, not absolute offers, that Jeremy would go in 2018. It was very delicate because I had not been authorised by anyone to negotiate, so I didn’t know who I was negotiating for. I was trying to stop a damaging leadership election.” Hold on: are you telling me McCluskey has the clout to decide when Corbyn goes? “I suspect he was using a bit of licence himself. He was caveating it, but it was certainly dangled.”

At the 2011 conference with Len McCluskey. Photograph: Rex Shutterstock

Does he believe he was an honourable deputy? “Oh God! Well, I hope so.”

I ask Watson if it’s true that he voted for Owen Smith in the subsequent leadership challenge. Watson seems taken aback, and stutters to an answer. “Erm, I never said who I voted for. Nor did I endorse any candidate.” Another pause. “I did vote for Owen, but I’ve never said it publicly before.” Why him, rather than his boss? “I thought, as soon as the leader loses the confidence of the parliamentary party it’s almost impossible to see how you can form a government. I thought Jeremy should have resigned, and he nearly did. But actually I was wrong on that. He went over the heads of the PLP and won a resounding mandate from the membership.”

Watson is still thinking about whether he behaved honourably. “I didn’t nominate Owen, which caused some irritation to his team. Nor did I publicly endorse him.” I burst out laughing – blimey, asking you to nominate Smith while you were Corbyn’s deputy is a bit much, I say. Now it’s Watson’s turn to laugh. “Well, they were feverish times. And you know, I did think about it – not nominating anybody seemed a bit cowardly.”

As the Brexit process wore on, Watson says he tried to voice his opinion privately. “I raised my view that there should be a people’s vote at the shadow cabinet Brexit subcommittee in 2018, and I was never invited back.” Whose decision was that? “I’ve no idea. So I decided to say it publicly, and we moved to a position in favour of a people’s vote, albeit a slightly nuanced one. I think I contributed to a policy decision that was pretty fundamental to the party and I was perfectly within my rights to do that.”

‘I’m not a great drinker. But I liked drinking a lot,’ says Watson, pictured at Glastonbury in 2016. Photograph: Tom Watson / Snapchat

In September this year, just before the Labour party conference, Watson was the victim of what he described as “a drive-by shooting” when Momentum founder Jon Lansman tabled a motion to scrap the post of deputy. Watson survived, and the motion was withdrawn, but he looked very isolated. Was he surprised by the motion? He was astonished, he says. “I don’t think you could pre-empt such political idiocy, and collective self-harm.”

But you do have a reputation for being serially disloyal to your leaders, I say. Actually, he says, he prefers to look at it another way: that he is non-tribal. “A tribal politician is somebody who stays within a particular faction and takes a line. I guess occasionally I’ve been a contrarian, more than disloyal. I’ve never really taken kindly to being told what to do.”

I ask Watson if he thinks he has changed over the years. “I was a bit shrill and immature in my early days. I’m much more aware of my frailties. I don’t feel the need to hide my inadequacies.” People have called him a bully – is that fair? He thinks long and hard. “I don’t think I’m a bully.” He doesn’t sound totally convinced. “I’m not a bully, but I’ve got a tendency to get obsessed with certain issues and not let them go. A dog with a bone is probably what I am.”

***

Watson already refers to himself as an ex-politician, and he clearly feels liberated. He is taking his level two gym instructor qualification, and admits it’s a struggle – he is older and less supple than the other students. But he insists the career change is real. He wants to educate people on the risk of diabetes and how to treat it through diet. What if he were offered a place in the House of Lords?

“No. I’m going to be a level two gym instructor.” Is that a promise? “Well, has it been offered and have I asked for it? No. And I think it’s highly unlikely that I’d be on Jeremy’s list. But were it offered me, I don’t know what I’d do.” Is that a yes? “I’ve got redundancy pay till 12 February. I haven’t even written a CV yet. I don’t know what my future holds. What I do know is that my days of frontline politics are definitely over.”

He returns to the heady days of 1974, when he was seven years old, watching his parents fight for Harold Wilson, and 1979, when he was campaigning for Jim Callaghan in a classroom mock election. That buzz has long gone, he says. And for much of the time since it has been all sound and fury. “Politics was like a cacophony of voices in my head for 35 years. And if I’ve got Jeremy to thank for anything, it’s that those voices are extinguished. I just feel free. I want to be an instrument for good. But I want to work with relentlessly positive people I like. And it’s very hard to say I’ve worked with relentlessly positive people in recent years.”

• Tom Watson’s Downsizing is published on 2 January by Octopus at £14.99. To buy a copy for £13.19, go to guardianbookshop.com.

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