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But for the snap election, Sir Vince Cable would have been in training this weekend for the national ballroom dance championships.

Instead, he is in a tiny office overlooking Big Ben, poring over his first party conference speech as leader of the Liberal Democrats.

“It was all mapped out carefully,” he sighs of his pre-election life, which included an LSE professorship and his first novel, which came out last week.

“I made a commitment to the local party to stand if there was an early election, never expecting there to be one.”

At 74, Sir Vince looks slimmer and fitter than when he sat in the Coalition Cabinet. He boasts of visiting the gym “several times a week” as well as cycling and climbing.

And he clearly relishes being back in the political skirmish.

“Overwhelmingly, the view is we are in the right place on the big issue of the day, which is Europe,” he said. “We want an ‘exit from Brexit’. It’s very clear and puts us in a different place from the Tories and Labour.”

However, while Sir Vince’s big pitch sounds clear at first, he becomes as woolly as his favourite cardigan when asked how his flagship pledge — a second referendum on Brexit — would work in practice.

The policy, he says, is for Britons to vote around the end of 2018 to choose between the final withdrawal deal being negotiated by David Davis or cancelling Brexit.

The first question is whether Britain has any power to stop the countdown to its exit following Theresa May’s triggering of Article 50.

“I know lawyers argue about that but I sense that the European Union countries would have no problem,” he said.

“I talk to people on the other side of the channel and ‘the Prodigal Son is welcome back’ is the sense that I get.”

His policy, it seems, relies on the permission of EU leaders.

What of the comments made in Brussels that Britain would have to opt into the single currency, as new joiners are obliged to do? Would that be a price worth paying? “No, I don’t think that’s remotely on the agenda,” said Cable.

But he conceded it could be “an issue” if the UK tried to rejoin after leaving in March 2019.

Would “perks” like the rebate won by Margaret Thatcher have to go, as stated by senior MEP Guy Verhofstadt?

“This is thinking far down the track,” said Sir Vince, before conceding: “A lot of these things would have to be renegotiated and it would be messy, clearly.

“You may be right that people might choose to be difficult but I am optimistic.”

He said the mood in European capitals was for a “multi-speed Europe”. When it was pointed out that Jean-Claude Juncker’s grandiose speech yesterday set out the exact opposite, Sir Vince savaged the European Commission president’s approach.

“He is absolutely in the wrong place and he does no favours for the British pro-Europeans,” he said.

“He happens to be a very bad choice for that particular job. He represents an extreme federalist position.”

Michel Barnier, the EU negotiator, was a “far more subtle” operator who ought to be commission president.

Sir Vince also thought Tony Blair was a liability to the anti-Brexit cause.

The former Labour PM claimed at the weekend that Britain could curb immigration while staying in the EU. “He was more or less saying ‘I was wrong’ and it was just feeding the negativity around this subject,” said Sir Vince.

“He does alienate a lot of people in the Labour Party and makes it more difficult for the more sceptical pro-Europeans to shift, because they don’t want to be painted as Blair.”

Sir Vince avoids criticising his predecessor Tim Farron’s record, but he admits to “frustration” that the party is so low in the polls, flatlining on seven or eight per cent, and says it needs bold new ideas.

He agrees with Paddy Ashdown who wrote recently that the “ferment of debate” is lacking.

He is convinced the party can win back lost supporters, including students, and talked of swapping student loans, which soared when he was the Coalition minister overseeing tuition fees, for a graduate tax.

Would he go into government with Jeremy Corbyn in another hung parliament? Although once a Labour activist, Sir Vince claims he “cannot see the circumstances”.

After the toxic shock of the last coalition, he would rather negotiate with a minority government from the outside. “The most likely outcome would be an issue-by-issue approach,” he said.

If the next election is in 2022, Cable will then be 79. Is he too old?

“It’s what you feel and how you behave that matters,” he retorts, blue eyes twinkling. “I’m here to stay the course.”