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There's a hardcore of Corbynistas who regularly attack Labour’s deputy leader Tom Watson.

They threaten deselection; accuse him of war-mongering, disloyalty, of supporting austerity and of being a “Blairite”.

There’s abuse: “Get the f*** out of our party!” It must be weird to be harassed by these newcomers given that Watson has been in the Labour Party his entire political life. But he’s not going anywhere.

“Centrism is not dead in Labour,” he says. “One wing of the party needs the other for this broad church to be successful. Right now the tradition that Jeremy represents is dominant. But plurality is the only solution. It’s essential for everyone’s voice to be heard for us to be electorally successful.”

Nor should anyone be thinking of leaving, he says when I ask about rumoured defections. “Stay and fight. Fight your corner; build your case. Don’t stop. That’s what I’ve done for 36 years. You win some you lose some in politics.”

Anyway, he continues, pluralism in the party “will make Jeremy Corbyn a better Prime Minister”.

One can only image the frothing and eye-rolling these comments will provoke. Centrism is anathema to the hard Left — worse than fascism. To them the notion that Corbyn would need support from the centre or Right of the party is crazed. But Watson sees himself as a “unifying” figure.

A united party is what they’ll need to be if there is to be a November election: “It is a challenge for me and it’s a challenge for Jeremy. We have to work hard to hold everything together.”

We meet at the Labour conference in Liverpool, which is not as “divisive and edgy” as he’d feared, in part because of leaks around the Conservatives that there may be a snap general election. “I’m not sure they are accurate,” he says, “but there are obviously conversations going on.”

Actually, he thinks a general election is more likely than another referendum vote — a People’s Vote — on the outcome of the Brexit deal because in the current climate “it’s effectively a general election with a referendum thrown in, isn’t it?”

He has been “very pleasantly surprised” with the quality of debates this week — Corbyn’s Labour is never as good as when campaigning and Watson feels they are “ready to go”. Although, he adds, “It’s still midweek and can still go awry.”

Brexit hovers like a fog over conference. On Sunday John McDonnell, the shadow chancellor, suggested that if there was a vote on the final Brexit deal, Remain should not be an option.

Watson disagrees. “I think at this point we don’t rule anything out. If a meaningful vote of MPs hasn’t been achieved, the deal fails and there’s no general election, and if you get to the point where there’s a People’s Vote, then Parliament will have to decide [what’s on the ballot]. That would require a debate, an amendment and talks with other parties. The delegates today are pretty clear that they don’t want anything ruled out.”

How would he vote? “I voted Remain last time and I am likely to vote Remain this time, but I would reserve judgement until I see the deal.”

This is his 30th-odd conference. Back in the early days he used to be responsible for charging walkie-talkies used by security. He also remembers being part of a rowdy piano-playing group that woke up Tony Blair — then Prime Minister — with their singing in the bar in the Grand in Brighton at 3am.

This year everyone wants to talk to him about his weight loss. In just under 14 months he has shed more than seven stone and reversed his type two diabetes (“I say ‘remission’ not reversed because I know if I eat refined sugar again I’ll get it back”).

Since Saturday “literally two dozen people” have approached, either to congratulate him or to ask for advice about weight loss. Many are type two sufferers. “And there’s a sort of side conversation, an embarrassed shuffle: ‘I’ve recently been diagnosed’, ‘My blood sugars are this’, ‘How do I do that?’”

“Embarrassment” is one of the reasons Watson himself can’t remember when he discovered he had diabetes. He went into a shame-based denial. He opts for “roughly four years ago”. The diet started around the time he did a day of fasting to raise awareness for political prisoners in Guantanamo Bay.

He gave up refined sugar and microwaveable meals “with 27 different ingredients” and late-night curries. He gave up beer and was stunned to learn a serving of basmati rice contains the equivalent of 10 teaspoons of sugar. “I had literally been eating a plate at least three times week for 25 years.”

He no longer eats “the kids’ fruit gums”, or chocolate mousse after lunch. “I also avoid or restrict stuff with a high glycaemic index.” Instead he eats blackberries and raspberries, nuts and “meat and two veg” for supper. “Another rule is: real food not processed food.”

So he cooks more and exercises. Initially that was “humiliating”. Why?

“Because when you are a 22-stone middle-aged bloke trying to do a press-up in a park in Lambeth you feel very self-conscious. Basically they are all Olympians in the park — or at least they seem it. But it gets better.”

He started by walking up the 76-steps to his office. “The first week I had to stop for a break. People were looking concerned and asking if I was OK.”

One journalist used to call him Tommy Two Dinners. Another described him as “like 10 John Prescotts”. Did it hurt? “Funnily enough I’ve always felt comfortable in my own skin. I was OK being big, I just didn’t want to die.”

Then he demurs. “Actually Steve Bell in the Guardian: he always calls me a fatberg. I do wonder whether he would fat-shame a female politician. I’m not sure he would. I’m not going to pick a row with a cartoonist but I do think different rules are applied.”

More recently Bell has called him Fatberg Slim, “which is quite amusing”, he says. But he doesn’t look amused.

The weight loss has become a political issue for him. He is already thinking about Labour’s obesity strategy if they get elected. “People’s understanding of what sugar is in what food is really important,” he says. He even compliments David Cameron’s government: “I think people will look back and say ‘the sugar tax saved lives’ because it lead to the reformulation of sugar in sugary drinks — not enough yet but companies reformulated their recipes overnight — and started a process by which the Treasury can apply levies on harmful sugar content.”

Watson sees the problem as not dissimilar to smoking (it also has self-interested powerful lobbies behind it). He would extend the sugar tax and find a way of taxing confectionary. “Some figures say that we eat 140lb of refined sugar a year. That’s at poisonous levels.” He says the NHS are amputating 120 toes or feet a week as a result of sugar-related conditions, that it’s causing blindness, and that 60 per cent of 10-year-olds are obese and “when those kids hit 30 they are going to break the NHS. We need to stop that trajectory. And talk to dentists — teeth are falling out. We just have to make the case for people to eat less sugar.”

He hasn’t yet had a conversation with Theresa May about diabetes — hers is type one, and very different. But he does say that the monitor she wears on her arm — which continuously checks glucose levels without the need of a finger-prick test — should be available to all on the NHS.

These days a proportion of his time is taken up giving friends advice. He finds himself texting, “Yeah nuts are fine”.”They are literally sending me their shopping lists.” He thinks he’ll probably start writing it down in his weekly newsletter.

As the weight fell off him Labour Party members sent him their old clothes. One rang Watson’s agent and said, “I’ve seen him next to Jeremy; he looks like he’s sitting in a tent.” He wore those suits for a while. But colleagues note a new, modish style. “He looks like an advert for Fred Perry,” one teases.

Watson says it’s the first time in years that he’s splashed out on “new gear”. As a teenager, “I was into two-tone and The Specials in a big way. It’s nice to fit back into Fred Perry.

I'm the healthiest I’ve been in 30 years. I’m also the happiest I’ve been in 30 years. Also, my brain works better, it was like a fog lifting. It almost feels like my IQ has gone up.”

His memory is deeper, he says. He can retrieve words quickly. “And,” he looks bashful, “in a strange way I feel more tolerant and compassionate in the way I view the world.” Hold on, is he saying he’s become nicer? “I just feel I have more time to listen to people. Yes, I’m a nicer person.”

This is truly a revelation. Watson — famously — was a bruiser: the ultimate Labour henchman. It is no surprise in this respect that he shared a London house in 2010 with Len McCluskey, general secretary of the Unite union. Watson says they talked about “poetry, music and literature”. He’s not joking. “We both liked the Liverpool poets. Also Yeats. The greatest line is ‘but one man loved the pilgrim soul in you’.”

Did McCluskey eat microwaveable meals? “I don’t know. He would’ve eaten out most nights.” Certainly, “he never did the washing up”.

Did they watch Strictly? “No!” He laughs. “He complained about me playing video games on the living room telly — particularly Portal 2.”

Today their relationship is “irrevocably broken”. McCluskey has never forgiven Watson for voting against Corbyn in the second leadership election in 2016 and they have not spoken since — “literally not a word. I haven’t tried, only because I don’t think it would work. He felt that it was my duty to do what he told me to do. And my responsibility was to the Labour Party. And it’s a disagreement that will never be resolved.”

He does however get on with McCluskey’s close friend Karie Murphy, who works for Jeremy Corbyn, but used to run Watson’s office. “She’s tough and Labour,” he says. “Works hard; does her job well. But she’s also a very kind person.” He relates that his 13-year-old son was up at the weekend (he has a 10-year-old daughter too). “And he had to endure the NEC meeting. He came out looking very pleased with himself and I said, ‘What’s the matter?’. He said: ‘Karie gave me a fiver’.”