It is the hottest day of the year, apocalyptic chords are crashing around an empty church and George Soros is describing his messiah complex. The man who made $1bn in a day by betting against the Bank of England always thought he was set apart. As a child, Soros had what he calls “messianic fantasies”.

“I felt I was putting in time until I found my place in the world,” he says, his rasping voice a reminder of his central European origins. “I was ill at ease in the position in which I was.”

Today, however, he finds himself in a locale designed to nourish outsize egos - Mosimann’s, a dining club in a former Presbyterian church. It is staff training day and the place is deserted. But we are going to be served all the same, having booked a private room for £150.

As we drink at the bar overlooking what was once the aisle, a Wagnerian crescendo swells behind us. It is the kind of music you might want to invade Poland to. No wonder the establishment’s corporate clients find it so invigorating.

Soros, however, is in a different league - so engagingly Olympian that he affects absent-mindedness about his greatest coup - Black Wednesday, September 16 1992. That day, when Soros successfully bet the British pound would be devalued, broke John Major’s government, led to the election of the Labour party and entered political folklore as an unforgettable date. But not, it seems, to the man who made the most money out of it.

“Was it Wednesday?” he asks. “It was Thursday, I think.”

“Wednesday,” I confirm. “Definitely Wednesday.”

“Was it?” he asks again, seeming to distance himself from his former self.

Today, the 75-year-old no longer moves markets. He admits his Quantum Fund, so feared throughout the 1990s, is not a major player. But if anything his ambitions are now even greater. Soros has set up a network of charitable foundations that span the globe, prides himself on having helped the 2003 “rose revolution” that toppled Georgia’s President Eduard Shevardnadze, and has poured money into opposing the administration of George W. Bush.

He has also written nine books, most of them over the past decade, dedicated to his pet theory of “reflexivity”. Having just read the latest - The Age of Fallibility - I want to stave off the moment when we start exchanging abstract concepts.

Instead, as Soros nurses his Campari and soda, I ask about 1944, the year when the Nazis invaded his native Hungary and killed hundreds of thousands of Soros’s fellow Jews. Soros himself had to deliver deportation notices. And yet it is the year he terms the best in his life.

“It was undoubtedly the formative year,” he says of that time, when his father obtained fake identity papers that saved the family’s lives and those of many others. “I was very close to my father and he imparted his entire wisdom in a practical demonstration of what you have to do to survive. A lot of my subsequent adventures in the financial world and my philanthropic philanderings were very much influenced by him, by this.

“I learnt then that there are times when the normal rules don’t apply,” he says. “Also the fact that it might be more dangerous to be passive - it can be less risky to take risk.” He leans back, discomfited by the heat.

Soros is by turns imperious, grandiloquent and humble. He revels in some of the more gushing ways he has been described, such as “the man who broke the Bank of England” and the “stateless statesman”. Yet he also listens, nods and offers up unflattering adjectives for himself and his arguments when I ask whether he is not simply a billionaire playing at being a big thinker. “Arrogant,” he suggests at one point; “obscene” he volunteers at another.

I remark on the difference between him and his father, whom he calls the biggest influence on his life. Both were shaped by their experiences in world wars - his father in a Siberian prison camp during the Russian revolution. But afterwards Soros senior never sought money or power.

“In some ways he was broken by the experience,” Soros says of his father, a lawyer who pioneered the world’s first and only Esperanto literary magazine but wound up running an espresso stand in Coney Island in New York. “He avoided the limelight.”

There is a rustle behind us and Anton Mosimann, the chef and proprietor of the establishment appears, smiling, bow-tied, moustachioed. He, like Soros, has a philosophy - his website says that chicken should taste like chicken and fish should taste like fish - and he has written even more books than the billionaire.

“You’re not open today?” asks Soros.

“We have some food for you,” Mosimann assures us and before long we are led to the back of the church, down past a set of firedoors, to the Davidoff Room - a sponsored dining area intended to resemble the inside of a humidor.

This is not the first time Soros has seen the back of a restaurant. After he came to the UK in 1947 (his father dissuaded him from going to the Soviet Union), he had a variety of odd jobs while studying at the London School of Economics. At one time he worked at Quaglino’s, then a stylish London eaterie, and subsisted on profiteroles. He also spent time as a trinket salesman and a swimming pool attendant.

But it was his studies at the LSE that proved more influential - particularly his fleeting contacts with Karl Popper, the philosopher who preached the merits of an “open society” over the totalitarianisms of Nazism and communism.

What swayed Soros was Popper’s insight that, since mankind could make mistakes, societies should be receptive to new ideas rather than based on rigid doctrines. He himself went further, arguing that people are bound to be wrong.

This is where his talk of “reflexivity” comes in. The idea is basically that people’s misconceptions interact with reality - whether through driving down a currency or promoting an idea such as President Bush’s war on terror. Soros swears the theory helped make his fortune.

“I really have no problem with being rich,” he smiles. (It’s a big weight off my mind.) He says he allows himself minor indulgences, such as keeping a permanent staff in his London flat although he spends most of his time in the US.

He made his way to New York to work as a hedge fund manager in 1956 and his father joined him the same year. Two decades later, after he had made his first $30m, he had a midlife crisis. “I was knocking myself out. I really thought long and hard about what I needed more money for,” he says as he digs into an endive salad. “As part of that process I decided to set up the Open Society Fund.”

Soros calls the network of organisations he finances “a cross between a foundation and a movement”. It has subsidised ministers’ salaries in Georgia after the rose revolution, saved scientists in the former Soviet Union from starvation and seeks to promote government transparency, human rights and a free press. It has even supported the Hungarian zither players’ association.

But it is for making money that he is best known. Stories abound about Soros the financier - about how he lost millions in Russia and Japan, about how he always wanted to raise the stakes. His $1bn profit on Black Wednesday, for example, came because he had bet $10bn. Investing also brought him pain - because of his fear of losing what he had risked. Despite all of Soros’s talk of reflexivity, he admits he often sensed trouble with his investments because of an ache in his back.

He no longer has the same appetite for risk - partly because he wants to see his foundations endure after his death. That means the days are over when he gambled his entire wealth in a single day.

“It’s a relief not to be dependent on the market,” he says, adding that he sees his legacy as his books and his philanthropy. “The money is a means to an end; the end is a philosophy translated into action.”

Nor has he given up trying to win support for his “conceptual framework”, even though he long ago confessed that he could not make head or tail of his own writings.

Who are his books written for, I ask, prodding a fork into my beef tartare and watching egg yolk slide out of it. Soros tranquilly munches his salmon, stopping to remove a hint of mayonnaise from his thumb. “Students,” he replies. “People who are still forming their view of the world.”

Students? But the last 50 pages of The Age of Fallibility are a reworking of a 43-year-old text that even Popper, his old mentor, wasn’t much interested in.

“Well I have a sense that I haven’t got my ideas across,” he says. He says the book is an attempt to study US society, which he faults for re-electing Bush and hence, in his view, making the world a more dangerous place.

“I basically wrote it to clarify my own thinking,” he adds. “The ultimate audience, so to speak, is me.”

He also wants to change public opinion, and in the last US election undertook an anti-Bush speaking tour - something, I suggest, that may amount to a rich man’s folly.

“Since I’m a rich man, whatever folly I commit is a rich man’s folly,” he half laughs, half splutters.

And isn’t it strange that a billionaire should write a screed against US consumerism and the way business seeks to stimulate desires?

“I have been successful within the capitalist system,” he replies. “Who better qualified to criticise globalisation than somebody who flourishes within it?”

The meal has overrun its time. We grab an espresso and then he leads the way back up to the main building, sliding his way past what appears to be an empty dessert trolley. At the main door, Mosimann and his staff line up to say goodbye and Soros ambles to his unpretentious chauffeured Citroen.

Days later the bill still has not arrived. I have little idea how much our meal has cost the FT. But, by any calculation, it came to far less than Soros made during a tenth of a second on Black Wednesday. Or Black Thursday. Or whenever it was.

Mosimann’s Private Dining Club and Private Rooms, London SW1

1 x Campari and soda

1 x tomato juice

1 x ceviche

1 x endive salad

1 x salmon

1 x beef tartare

2 x glasses of white Burgundy

1 x bottle mineral water

2 x espressos

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