October has been a strange month for Vince Cable, the Liberal Democrat Treasury spokesman and fairy godfather of British politics. After his annus mirabilis as the “undisputed heavyweight champion of the credit crunch” — a quote proudly advertised on the jacket of his new memoir, Free Radical — all of a sudden it looked as if Cable’s personal boom might have turned to bust.

First, people began to question whether he had really predicted the crash in any meaningful depth. Sure, he’d been opposed to overspending, but was there anything more in it than a good-hearted liberal concern about people borrowing too much?

Then there was the “mansion tax” meltdown at the Lib Dem conference. Announcing an eye-catching plan to impose a levy on houses over £1m, Cable apparently failed to consult colleagues on the matter, leaving them to denounce it as “codswallop” and wonder whether his new-found celebrity had gone to his head.

Cable, 66, lets out a long sigh when I remind him of the mansion tax moment. “I think it’s the nature of party conference that you don’t have the opportunity to refine positions,” he says, sitting in his slightly cramped, cluttered office high above Westminster. “You have an opportunity for parties to set out their stall and produce new ideas for debate. Parliament hadn’t met for almost two months. It’s partly in the nature of the event but, okay, I could have done more to consult people.”

Inevitably the matter served to highlight something of a weeping sore: a supposed tension between Cable and his boss, the Lib Dem leader Nick Clegg. Back in 2007, rumour had it that Cable had vied for the top job and was rejected. Today he is quick to deny any further troubles.

“We have a very good relationship,” he insists. “We talk frequently. I’m not sitting waiting to take over, competing for his office.” So no Brown-Blair shenanigans? “Ha ha,” he laughs, exposing two Shrekky front teeth. “No, I haven’t got Brown’s temper.” Still, isn’t it awkward for Clegg?

“I don’t think so,” he says. “He’s getting very good ratings himself. I was active when Paddy Ashdown came in, and when Charles Kennedy came in, and actually Nick’s become recognised much earlier in his tenure.” Besides, he’s not going to stop holding his own opinions. As we talk, the cameras are rolling on Gordon Brown in Brussels, speaking in support of Tony Blair as a possible president of the European Union. Clegg has denounced Blair as a candidate, but here’s Cable musing on the same subject: “I’m not partisan, but there is an argument for having someone who sticks up for Europe, a well-known figure that the president of China will recognise and take seriously. If you want Europe to project on the world stage, you need Blair.”

He is disappointed, of course, that we have not emerged from the recession yet — as America did on Thursday — but he is “not surprised”, pointing out that he has long since spoken of Britain’s greater vulnerability to financial problems. Cable is understandably proud of his foresight, especially the “much-quoted exchange with Gordon Brown in parliament”, he says, referring to a moment in November 2003 when Brown, then chancellor, famously dismissed his warnings about overspending, an exchange on which Cable’s status as the sage of the crash was largely built.

In reality he has been somewhat inconsistent on the matter. In the same year, for example, he dismissed the early warnings of the International Monetary Fund and he arguably contradicted himself recently over the fiscal stimulus. In October 2008 he initially supported the HBOS-Lloyds TSB merger, only to change his mind a few months later.

“It seemed to me the right thing to do in the middle of a panic,” he says now. “And then when you look at it ... Almost certainly [Brown’s] now going to have to unravel it. So that was one thing. Some of the other things I was quoted as having changed my view on were not actually right. A remark I made about quantitative easing . . .”

“The Robert Mugabe school of economics”? “That was an attempt to be ironic,” says Cable. “It was interpreted as an attack on that kind of approach, but on the day it happened I rushed out to support it and, indeed, did support it in everything since.”

Even if he does like a swift, smart soundbite — he memorably described Brown’s fall from grace as “from Stalin to Mr Bean” — Cable is hardly impetuous. His own rise to prominence has occurred at an almost geological speed; he doggedly fought and lost five general elections, first for Labour, then switching to the SDP, before finally being elected for the Lib Dems in Twickenham in 1997. He describes his current party’s battle as a “war of attrition”; this may equally be applied to his own career. The role of not-leader, he says, “gives me more time to write books and be with my family”, although given that he describes himself as a “workaholic” and admits his wife reckons his work-life balance is totally out of whack, this doesn’t ring true. But at least he has had the chance to write his memoirs, an account of his journey from Nunthorpe grammar school, York, via Cambridge and the oil giant Shell, to the House of Commons. He is frank about various adversities, including his mother’s mental breakdown, bouts of domestic violence and his controversial first marriage at 23 to Olympia, a beautiful Kenyan Goan he met while working at a psychiatric hospital.

Back in the 1960s, mixed-race marriage was almost unheard of; both sets of parents were utterly opposed. But they got married regardless and had three children. I wonder if his “total indifference” to his parents and the little emotion he felt at their deaths stemmed from their initial intransigence or if it reflects something else about the man. When Olympia died of cancer in 2001, he admits he viewed her passing with a certain emotional detachment. “It was difficult,” he says, “but I don’t want to exaggerate. It’s the patient coming to terms with the end of their life. I was one stage removed, the carer.”

Anyway, within five months a second Mrs Cable was in the viewfinder. Olympia died in May and he went to India to console relatives. There he made a decision: “I’m going to get on with my life. I restarted my dancing classes, having a social life, and I met Rachel [Wenban Smith, whom he married in 2004] very shortly after, in October.” He describes her as “an attractive middle-aged woman whose elegant legs, enhanced by tight jeans, I kept noticing . . .”

“Seen from the outside,” says Cable of the timeframe, “people react as you do. Unseemly haste. But I didn’t think of it like that; I grieved for Olympia while she was alive. Once I’d been to India and cleared my mind, I thought: what shall I do to start enjoying life?”

What did his three children think? “They were cross with me,” he says. “It was quite difficult for them. They were around while she was dying, especially my younger son, who took a year off university, but it took longer for them to adjust. Initially it wasn’t easy but we’re now a family, a united family.”

The children had their reservations about the book, too, but “I think in the end they were happy” and asked him to take only small things out. Well, there is a definite hard streak of defiance, a bullishness, to Cable; not only in the dealings with his family but at work, too. His pride in his role as chief economist of Shell in the 1990s, for example, a time when the company was allegedly accountable for human rights violations in Nigeria, is immutable.

He describes himself as something of a puritan, especially when it comes to money. He went into politics to sort out the economy. I wonder how far he would go to untangle the present crisis. If, say, the Lib Dems don’t get into power at the next election, would he consider moving to another party to see his economic plans come to fruition?

“Not as an individual,” he says, although he admits that he has been informally approached by both parties. “We’re part of a team. It’s not like Carlos Tevez jumping from Manchester United to Manchester City. I’m part of the Lib Dem team, we do well, we go into government or we have a role.” Well, what about as a party, then? A number of scenarios are possible at the next election, including a hung parliament. “We’ve made it very clear we’re not in the business of picking winners in other parties.”

For the moment he’s happy to stick with books, trips and his enduring love, dancing. His ballroom skills are famous: right now he’s learning a new version, the Argentine tango, and he shows me a few steps. “My wife is learning, too, but I only manage about an hour a week. It’s quite a sexy dance, actually. Do you know it?”

Free Radical: A Memoir is published this week by Atlantic Books, £19.99

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