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Nick Clegg is an unlikely revolutionary. If anything he has always been the face of dull, moderate politics. No one can imagine him losing his temper; he wears V-necks over open-collar shirts; uses descriptions like “naff” and “a pig in brown stuff”. He even wrote a book entitled Politics: Between the Extremes. Everything about Nick Clegg screams “Centrist Dad” (the Corbynista insult for out-of-time metropolitan liberals). But a lot has happened to the former leader of the Lib-Dems since he disappeared from the Westminster stage through the trap door of the June general election.

For a start he is angry — angry about Brexit, angry about the stagnant state of politics, a little bit angry at the world. He has channelled this into a radical handbook called How to Stop Brexit. It instructs fellow angry, politically impotent metropolitan liberals — who have lost all their power to the extremes of Left and Right — to stage a coup using the revolutionaries’ own method: mass entryism. “If every one in 100 Remain voters joined the Conservatives,” he argues, “they would more than outnumber the membership.” Those who can’t stomach being a Tory — even in name only — should infiltrate Labour. “It’s what Momentum did to Labour, it’s what Aaron Banks did to the Conservatives.” Clegg the unlikely revolutionary has issued a call to arms: it’s a middle-class momentum, the alt-bourgeoisie.

At 50, Nick Clegg looks older than when I met him four years ago as Deputy Prime Minister. This is not the famous accelerated ageing process that goes with being in government — from that he emerged well preserved. It’s because this has been a year of hell. Last autumn his 15-year-old son Antonio was diagnosed with lymphoma (blood cancer) in his neck and chest. Nine months later Clegg lost Sheffield Hallam, the seat he’d represented for 12 years.

He wrote most of How to Stop Brexit in July, in what he describes as “compressed time” but sounds like a torrent. The motive was to debunk the “pernicious myth” that a democracy is not entitled to change its mind. “I deliberately put at the front page of the book the quote from David Davis that says: ‘A democracy that can’t change its mind ceases to be a democracy’.”

He follows with a quote from John Maynard Keynes: When the facts change, I change my mind.” The Brexit we face, he argues, bears no resemblance to “Leave”. “David Davis said we would have trade deals equivalent to 10 times the size of the European Union. That is statistically impossible.”

The book also makes the “very specific proposal” to ask John Major and Mark Rutte, the Dutch prime minister, to set up a new convention to re-integrate the UK into a reformed European Union, an idea worked on at Open Reason, the think tank he set up in 2015.

I’ve come to his home in Putney, where he remained while in government with his wife, the lawyer Miriam González Durántez and their three sons, Antonio, Alberto, 13, and Miguel, eight. It’s a pretty Regency townhouse which has just been painted today — for which Clegg needlessly apologises.

In the loo there is a paperback of UK citizenship tests entitled How English Are You? And in his office, huge windows overlooking the street, someone has colour-co-ordinated his books: red on one shelf, blue on the next, then yellow. When I point this out he swivels in his chair and takes fright. “I haven’t done that. That has happened miraculously. I find that alarming.”

Apparently the family were offered a flat in Admiralty House, a “socking great big thing overlooking St James’s Park”. And Home Office officials were keen he take it, reasoning that David Cameron and George Osborne had moved into Downing Street. He hadn’t finished the sentence suggesting the move when Miriam flat-palmed him.

“And I said, ‘But darling…’ And she said, ‘No.’ I went back the following day and said, ‘My wife said no.’ They said, ‘But surely Deputy Prime Minister…’ And I said: ‘No, you don’t understand. My wife said no. That means no.’”

Miriam’s argument, “quite rightly”, was that the children needed the “cocoon” of normality, of going to the primary school at the end of the road, of maintaining local friendships. Their home was overhauled by security and police were dispatched. The phrase “Miriam says no” became a useful shorthand in the office. “They knew it had a certain sort of absolutism about it.”

The commute to Whitehall “nearly bloody killed me”, but it was worth it. “We talked about the children, but never used them as political ornaments.”

Indeed, when he and Miriam were recently interviewed by ITV’s Lorraine Kelly to raise awareness for the charity Bloodwise, Antonio, now in remission, suggested he go on too. “We said: ‘No you’re bloody well not. You’re staying here and doing your homework.’”

He describes the terrible helplessness when you discover your child is so sick. “The most stressful moment is the diagnosis. When you first hear the ‘c’ word it’s like a bomb. It’s an almost unhelpfully paralysing term. Then you go online — urgh — and start chasing the worst possible outcomes.”

He felt an “irrational” but visceral urge to take on the disease himself. “Every fibre in your being is saying: ‘Can I take it. Give it to me.’ Then you ask why and of course there’s never an answer.”

Tapping the wood of his desk he says that the treatment for Antonio seems to have been successful. “But it’s brutal. You have chemicals poured into you for half a month, then half a month off to recover in order to take the next blast. It’s napalming the body to kill the disease.” Side-effects were “horrible. At one point he was neutropenic. He lost his hair; his appearance changed. Your glands get brittle, he was taking 21 pills every day.”

Clegg felt more useful once immersed in the routine. “Pills, scans; the back of the kitchen door was adorned with all these charts and graphs — is he taking the right pill at the right time? Is it me or Miriam taking him to UCH this time? Those practicalities make you feel like you are doing something.”

Antonio pops in to say hello. His hair is back and he’s a handsome 6ft 3in, with feet so long, Clegg jokes they have to buy him special shoes. “I said to the oncologist the other day: ‘Did you put plant fertiliser in the chemotherapy? He’s shot up.’”

Antonio’s brothers were matter-of-fact about his illness. “Their attitude was ‘as long as he is going to get better’. They still teased him. He was still the bossy older brother. They are amazing kids, much more straightforward than adults. We tie ourselves up in knots.”

With all this going on, I can’t imagine how he fought his seat. “With hindsight I wasn’t as focused on the campaign as you need to be,” he says. About the result he is fairly sanguine, saying the real kick in the teeth happened in 2015 when the Lib-Dems were effectively “whacked”. This time his heart hadn’t really been in it — in part because of Antonio, but also “I was never much of a Commons man”.

“The Jacob Rees-Moggs of this world love the pomp, the ceremony, the leather benches, the wooden panelling. It reminds them of high tea. None of that ever appealed to me.”

Actually he is “bored with the bullshit of Westminster”, with the “equivocation” and the “messing about”. “Do you know why I like Alastair Campbell so much?” he asks. “His fearlessness. He just goes for it. I love that. It’s rare in politics.”

Emboldened, he supplies a savage account of May’s time as Home Secretary. He saw her more than other politicians: they sat next to each other in Cabinet, at the National Security Council, and they met regularly, aides in tow, to discuss the many “flashpoints” between Home Office and Liberal Democrat policy

And though she was “punctual” and “polite”, he struggled with her manner. “Maybe I’m too much of a touchy-feely person, but there was just no...” He massages the air for the word to describe this nebulous feeling but — perhaps ironically — finds nothing. And her aides were “irritating”, he continues. “They would speak in her stead, she would defer to them, or they would interject.” But it was worse after he requested meetings without them. May was exposed. “She didn’t have the confidence or the wherewithal — I’ve never to worked out which — to say ‘Yes, let’s do that.’ Time and again she’d have to go back to the office, consult [with aides], then write a letter. It just took soooo long. It drove me crazy.”

About Boris Johnson, too, he is scathing. He accepts that Boris is “pretty clever” but sees him as a greatly “diminished” figure. When he was London Mayor, he says, people “would scurry around saying: ‘We’ve got to give Boris whatever he’s asking for.’ But I don’t think people are afraid of him any more. He’s become a media phenomenon.”

I badger him about the future of the centre ground and he jokes, “I’m Centrist Dad with an ideological paunch.” But later he revises this: “The British liberal tradition is rich, muscular and self-confident. It’s not a split-the-difference insipid ‘Centrist Dad’; it’s optimistic, reforming and restless. I’m all for vigorous centrism.”

Would Clegg return to politics?

“No.”

Never?

“Well who knows? I’m only 50, maybe when I’m 60.”

How to Stop Brexit (And Make Britain Great Again) by Nick Clegg is published by Bodley Head on Thursday