Pi Capital's boss has questions for David Cameron and Nick Clegg

The head of Britain's hottest salon-cum-think-tank says his perfect guest would be Warren Buffett

It was standing room only when around 40 entrepreneurs crowded into a private room at the Lanesborough last Tuesday to hear John Kay, a leading economist, expound on his radical views that banks should be split up to avert another financial catastrophe.

Even David Giampaolo, who organised the breakfast, was quietly delighted that so many people turned up for such an esoteric subject as "narrow banking" so early in the morning. His guests were ahead of the game; the new Business Secretary, Vince Cable, is another bank splitter – and setting up a commission into the banks was one of the first actions of our new coalition.

Giampaolo shouldn't really have been surprised by the turnout as his Pi Capital events are the hottest dates in town. The night before Kay's talk, another 60 or so like-minded people were the fitness entrepreneur's dinner guests, and heard Lord Coe and Sir Keith Mills update them on the Olympics. Tomorrow he's snared Guy Laliberté of Cirque du Soleil to explain to Pi members over lunch how he turned his stilt-walking circus act into a billion-dollar empire. Squeezed in between, the American-born 51-year-old has also dined with the Swiss, Maltese and Egyptian ambassadors, FTSE directors and the London Stock Exchange's Xavier Rolet.

There is just about room for the two of us when we meet in Giampaolo's office, hidden away in a rabbit warren at the back of Berkeley Square. He's clearly not one to waste a penny, and runs Pi, a private investors club, with a staff of five. Piled precariously on every possible surface in his minuscule room are books, magazines, papers and cartoons, not unlike a newspaper office. Like a magpie, he's a one-man news aggregator; his trademark FYI emails bringing the most eclectic and thought-provoking mix of research and articles which he collects from around the world to those in the magic circle. You can soon see why they're so drawn to him: he's curious about everything and fizzes with ideas.

It's Tuesday, the day Gordon Brown resigns as Prime Minister and David Cameron announces his coalition with Nick Clegg, and he has questions for them on how to stimulate growth in the economy. "Where's the wealth and job creation coming from? I don't see it. There isn't a quick fix, but unless we find ways of encouraging new business and supporting small ones, our problems will get worse. We need a mix of tax-breaks, incentives with some banking and government support," he says, adding that we can't rely on the banks for funding. "Politicians bashing the banks, forcing them to lend, doesn't work because they need secured assets – and so many new businesses don't have that security. It's mad, but it's a fact that it's more difficult to raise £5m than £50m – we need to solve that."

Pi members do what they can. About 200 of his 300 members have backed Pi-related ventures and more than £150m has been invested since Giampaolo bought the business eight years ago. While the more well-known members include Lastminute's Brent Hoberman and Sir Stuart Rose of M&S, in the shadows there are Norwegian ship-owners and South African glass billionaires who want their money to grow even more. That's when Giampaolo acts as a sort of Gabriel to the angels: "I get new ideas to back coming to me every day of the week, which I filter first. I give people a ring, say I'm looking at this idea, do you want to take a look? They decide pretty quickly and off we go."

He winces when I ask if he's really got the best black book in London: "I've got a great network, yes, but it's not the best. That's not what I'm about, but I do have 300 really interesting investors who have access to each other, provide each other with information, and who get even better access from our social events. By the way, no one sits next to anyone at my events by accident; there's always a reason."

Giampaolo's own story is a rags-to-riches tale. His grandparents arrived penniless in Philadelphia from Italy at the turn of the century. But his grandfather died young, so his own father left school at 12 to help support the family, although he went on to play the saxophone for a living. "My nana had to look after five children, earning a nickel a day for her sewing. It was tough; they walked to school in the snow without shoes. That's why I was so surprised when my pop didn't say I was nuts when I told him I wanted to leave school at 16.

"He knew I was bored, and although he didn't stop me, he did order me to leave home to become a man. I moved out, traded in my Gran Torino for a van to fit in the lawnmower, and started cutting grass around the neighbourhood. It was pretty good, one in 10 said yes." He laughs at the memory. Florida's sweltering heat soon got to him though, and he headed for the local gym where he became an instructor. Soon he was the manager, setting up his own gym with $3,000. A few years later three doctors – who worked out at the gym – lent him $50,000 to start another fitness centre, with air-conditioning, and off he soared, setting up and selling a number of gyms around the US.

He's been in the UK for 20 years, opening a centre at the Barbican for America's biggest gym group – Princess Di was at the opening – and off he went again to co-found Fitness Holdings Europe. Then he helped BC Partners in the £835m buyout of Fitness First, making his own small fortune.

Looking for something new to do, he came upon Pi, then a small investment club. Backed by funds from a small family office, he turned it into today's think-tank-meets-19th-centurysalon, and is now hoping to roll Pi out in Mumbai, Dubai and Asia.

He's hosting another 20 or so dinners in the next few weeks – including ones with Jed Bush and Iain Duncan Smith. Such a frenetic social whirl doesn't leave him much down-time, but then Giampaolo doesn't do much down-time: "Work and social is insanely seamless. But I love it, although I do try to eat less and exercise more at the weekend: my children make me 'go, go, go....' "

Who would be his perfect dinner guest? "Warren Buffett. But I've never asked; I couldn't bear the rejection." Usually he gets who he wants and for free, mainly because the quality of the Pi people is as high as the invited guests; "My guests know they will get really robust debate – not always from people who agree – but certainly with rigour," he says. "The first thank-you letter is always from the speaker."

Has anyone ever turned him down? "Only one out of 200 in five years – it was Gordon Brown, the then Chancellor," he chuckles. Hours after we meet, Mr Brown has gone. Perhaps it's bad luck to say no to Giampaolo.