The thing about Stuart Wheeler is that he seems perfectly harmless, a snowy-haired giant in a shambling suit. Peering out from square-rimmed glasses, the 75-year-old doesn't strike you as a multimillionaire card shark, the man who invented spread-betting, and whose firm IG Index netted him £90m 10 years ago.
Charming and witty, he is one of those embarrassed Old Etonians. He dines at White's, he's good at bridge ("I'm about the two- or three-thousandth best player in England"), and he divides his time between Mayfair and Chilham Castle, a Jacobean jewel in Kent that he bought nine years ago. So far, so P G Wodehouse.
But then, you come to Europe. For reasons that never become entirely clear during a two-hour conversation in his Berkeley Square penthouse, Wheeler has a Basil Fawlty-ish hatred of the European Union. It destroyed our fishing industry, he says; it's a massive drain on funds; and we've never got anything out of it. Britain, he says, must withdraw at all costs. And this is not just some wistful golf club fancy; his conviction was made flesh last week when he was appointed treasurer of the UK Independence Party.
The move is a massive boost for his friend Nigel Farage, ahead of the 2014 European elections, in which Ukip now has a strong chance of making gains. It is also a major blow to the Conservative Party which, only 10 years ago, was enjoying Wheeler's support in the shape of a cheque for £5m. It remains the biggest single donation to a political party in British history.
To have lost access to Wheeler's deep pockets seems, by any standards, careless. He was expelled from the party in 2009 for donating £100,000 to Ukip. According to him, Eric Pickles, then the party chairman, sent him an email to inform him of this, though Pickles claimed he told him on the phone. "Why lie about something so unimportant?" snorts Wheeler.
Whatever the truth, the loss to the Tories is more than just financial. More damaging is Wheeler's very vocal disenchantment with David Cameron, who, he says, has betrayed the right wing. "Before the election, everyone said Cameron can't come out as being too Eurosceptic. They all said: 'Just you see what he does when he comes to power.' Well, he's there now, and he's made no efforts whatsoever to curb the influence of the EU. He has given up, and it's disastrous for our country."
The trouble for Cameron is that Wheeler is well connected, in business and in the same smart society circles in which he himself moves. Wheeler played bridge with Lord Lucan two days before he disappeared, and Omar Sharif was once a partner. His wife is the photographer Tessa Codrington, and they have three daughters, including the model Jacquetta Wheeler. And he clearly feels he is not alone in being disappointed with Dave. "Mervyn King said Cameron and Osborne made their judgements almost entirely on electability. I think it's quite fine that that's come out."
But here, perhaps, is the rub. For, while the views of the Governor of the Bank of England on the suitability of the Prime Minister are of some national importance, those of a spread-betting tycoon probably aren't. At least, they probably shouldn't be. And yet, of course, they are. Ninety million pounds can go a long way in politics, as Wheeler is happy to admit. "I wouldn't have got involved in politics if I hadn't got this money," he says. "It's only since then that I've started going to think-tanks. And it has been terribly interesting."
You get the feeling politics is all a bit of a laugh for Wheeler, something to keep him amused in his retirement. "Whatever makes you happy, Daddy," was the reaction of one of his daughters when told of his new job last week. When he made the £5m donation, another of his daughters burst into tears, though it was the money that she minded more than giving support to the Tories: "I was like, 'What? What have you done? What a ridiculous, ludicrous thing to do. Think of the dresses I could have bought!"
Actually, they probably have all the dresses they need, as Wheeler has been a generous father, though there's no guarantee there'll be much left to inherit. Gambling is still his hobby, be it online poker or bridge weeks in Tangier, where his wife inherited a house. In the end, he puts his wealth down simply to good luck. In fact, he's not that effective at managing his fortune, which is down from £90m to £40m after some ill-timed share sales, and the purchase of his "so-called castle". He says the cost of running Chilham means his income is less than half his outgoings. "I completely underestimated the cost of refurbishing and furnishing it, and the 23 per cent capital gains tax. So I don't feel rich, because it's eating into my capital the whole time. Not that I expect anyone to shed a tear for me."
How he came to be sitting in a multi-million pound penthouse beneath a portrait of Margaret Thatcher, which he bought for over £250,000, is an extraordinary story. Born to a 42-year-old spinster in 1935, she gave birth in secret and immediately put him up for adoption. He was brought up by an American banking heir, Alexander Wheeler, and his wife Betty Gibbons, the daughter of a baronet.
Although he always knew he had been adopted, he researched his real mother only late in life, once she was dead, eventually meeting his surviving relatives in a poignant reunion in Edinburgh. A film of Chrissy, his mother, had been found, showing her walking bolt upright with her hands behind her back – just like her son. It turned out that she too had been a lifelong bridge player.
Wheeler's public life only really began, as he knew it would, with that £5m donation. After Eton, he did national service in the Welsh Guards before reading law at Oxford, working as a barrister and a merchant banker before setting up IG in 1974. Politics became his hobby in 2000, when he was introduced to William Hague at a dinner at White's and, impressed by the young leader, made a donation of £15,000. Soon after, in the run-up to the 2001 election, Labour received a couple of million-pound donations, and, seeing the Tories' finances were in a pitiful state, he decided to step in. "I had just got all this money, and I remember thinking, what difference does it make if I have £90m or £85m? Whereas, £5m made all the difference to the party."
It's in this blow-with-the-wind whimsy that Stuart Wheeler emerges as a potentially dangerous figure. He once said that the influence donors wield on politicians is "absolutely natural and unobjectionable". When I ask why money should buy access to the Prime Minister's office, he shrugs and says, "That's just how life works." So it's alarming to discover that his ardent Euroscepticism is not born of a long-held ideology, but is rather a new acquisition. In the 1970s, for instance, he voted for Britain to stay in the Common Market, and he only became positively anti-Europe "in the late Ninetiess, or maybe the early Zeros, or whatever you call them."
Vagueness is something of a conversational tic, masking, one would imagine, an equally fierce intelligence. Sometimes, though, it's a little disconcerting. Does his anti-Europeanism mean he is pro-American? "I don't know. I haven't really thought about it," he says. At other times, it doesn't quite ring true. When he gives directions to the flat, he says it's in the same building as "that Italian restaurant all the smart people go to". What, Cipriani? "That's the one." Could he really not know the name of the restaurant in his own building?
To get a clearer picture of where Wheeler is coming from, I turn to his pamphlet, A Crisis of Trust, published last year in the wake of the expenses scandal. While most people might satisfy themselves by writing a letter to The Telegraph, Wheeler dashed out a whole treatise, explaining how trust in politicians had been eroded by the creation of a self-interested political class. He also used it to explain his position as a climate-change denier. Maybe he's right, but where does all this conviction come from?
His arguments are compelling and forcefully made, but they're not always consistent. In the pamphlet, for instance, he wrote: "I do not necessarily want to leave the EU: there are problems and issues if we do." Today, though, he has changed his mind and is adamant that withdrawal from Europe is the only solution.
And then there is climate change. He argues that global warming is a hoax got up by irresponsible alarmists. He cites the case of the leaked emails from the Climate Research Unit at the University of East Anglia, in which scientists appeared to have manipulated data to support their theory. But he doesn't deny that the earth's temperatures are rising. And when I point out that all the main political parties agree on the existence of climate change, and that it's only in the world of conspiracy theories where you find claims that a darker force is frightening us into believing it, he agrees that maybe it is. "But has Cameron read the facts? I don't think he has. He's a very busy man."
Maybe he is right about Europe. And maybe climate change is a hoax. He is certainly convinced about both those things, or at least he is today. But who knows what he might believe tomorrow?
1935 Born 30 January in Harrow, north-west London. His mother, Chrissie Cleland, 42, puts him up for adoption.
1937 Alexander Wheeler, a 55-year-old former Army officer and banking heir, and his young wife, Betty Gibbons, adopt Stuart and a girl, Susan, on the same day, 4 January.
1942 His adoptive father dies of leukaemia, and the family moves from Devon to a suburb of Oxford.
1957 After attending Eton and doing national service with the Welsh Guards, he graduates from Oxford with a second-class degree in law.
1974 After stints as a barrister and in merchant banking, Wheeler sets up IG Index, a company that allows UK residents to speculate on the price of gold, at a time when you couldn't buy it, except at a premium.
1993 IG branches into spread betting on sports, which becomes a huge industry.
2000 Flotation of IG Index earns him £90m in shares.
2001 Donates £5m to the Conservatives' election campaign.
2002 Sells shares worth £7.3m to help buy Chilham Castle, a Grade-I listed mansion near Canterbury.
2008 Brings legal case against the Government over its failure to hold a referendum on the Lisbon Treaty.
2009 Expelled from the Conservative Party after donating £100,000 to Ukip, in protest at Cameron's stance on EU.
2011 Appointed treasurer of Ukip.