The man with the lean stomach (I felt it and it's as hard as a water melon) is called Kisen and he teaches yoga. Nothing strange in that you might think - 400,000 people regularly take classes in Britain, according to The British Wheel of Yoga, a figure that's doubled in five years.
But Kisen, 41, will have nothing to do with the current yoga-fest. He disparagingly calls it "supermarket yoga" and the power yoga practised by Madonna is his favourite bugbear. "Two American guys turned up in India in the 1960s, discovered yoga and brought it back to the States," he says. "The problem is that they remembered the impressive poses but forgot the meditation and breathing."
Kisen teaches Ashtanga yoga, which puts the emphasis on meditation and breath - the 84,000 possible postures are almost incidental, he says. Too many yoga teachers don't show you how to breathe properly and yet, he says, the breath is the catalyst in any yoga pose. The breath energises the body. Without it you're simply wobbling on one leg and tying yourself up in silly knots.
In power yoga the breath is short - Kisen puts his head in his hands at the very thought of it. In Ashtanga yoga the breath is long and deep. He'd win a competition for blowing up balloons - he can inhale for one minute, exhale for another.
"In power yoga the softer aspects are forgotten. It's all too rigid and aggressive. It reinforces the idea that we've got to look good," he continues. This isn't to say that regular classes with Kisen won't improve your posture, strengthen those tummy muscles, loosen up the tightness along your shoulders. But his students claim that you'll actually feel better too.
Geri Halliwell contacted Kisen nine months ago, at the point when the fall-out from her break up with the Spice Girls was at its height. She was not a happy woman. Kisen, who nicknames her Gingernut says, "I've talked to her about slowing down, taking life easier, seeing the beauty in ordinary things." Now she appears sassier, more sussed, more relaxed. As for her figure, it's leaner, wirier, less bulky. The other day she rang him to say "I've just seen my new video and guess what, Kisen? I look like a babe!"
"Too many people are addicted to being perfect," says Kisen, reflecting on his more ambitious class members. "What yoga can do is teach them that they don't have to be perfect."
Tracy Brower, 29, runs her own public relations company, Brower Lewis. She started doing yoga classes with Kisen in 1997. She talks less about the way her body has changed in the last two-and-a-half years - although it is more supple - and more about emerging from a class feeling energised. "There's this incredible sense of freedom, of peace," she says.
When people first come to classes Kisen says he's "vindictive". "People think I'm being cruel. But I want to teach them that they don't have to be perfect. And also that the body is capable of doing anything it wants to."
Paul Brewer agrees. When he first turned up to a Kisen class he was an ex-rugby playing 18-stoner and a metre away from touching his toes. Accustomed to sitting at a computer all day - he had worked for a Wall Street investment bank - he hadn't sat cross-legged since the infants.
Two years later Brewer, 42, has made what Kisen describes as a "quantum leap" both mentally and physically. He's lost more than 30lbs and left the bank to set up his own company. The four colleagues who have joined him are also doing yoga now. All this and he's a dab hand at headstands.
"To begin with it was pretty uncomfortable," says Brewer. "My whole body was tight. But Kisen seems to instinctively sense where the trouble spots are in your body. And because the breathing energises you, I come out of classes feeling a million times better." He's not your traditional soak-lentils-overnighter but he says there's been a spiritual shift too. "The meditation side of the classes has helped me to become more centred. Not self-centred, but more focused. I needed a change of direction and yoga has had an impact on the way my career has changed."
Kisen himself is a complex mixture of influences, by turns self-deprecating and self-reverential. There's an evangelism about the way he talks about yoga, but on the other hand he'll finish a class with a cigarette and a beer. These opposites make sense when you know his background. The son of working-class Irish Catholics who ran a pub in Liverpool, he was born plain Eddie. Until the age of 30 he worked as an illustrator. "A typical routine for me was to work all night on a job, fuelled with junk food, black coffee and whiskey. I'd sit at my desk hunched over for hours on end. My only relief was a blow out in Amsterdam every few weeks." He had the most expensive BMW and limitless credit on his banker's cards. He was also, by his own admission, a hard-drinking bloke, always up for a fight, who went to the gym for one reason: bigger biceps.
Then one day at the gym he bumped into a man called Godfrey "who was skinny as a whippet", he recalls. They got talking and it turned out that Godfrey was a yoga teacher. Kisen went along to a class. Suddenly the biceps didn't count, and in a roomful of women he felt the weakest of the lot. But he was intrigued and he continued the classes. His life changed.
Ten years on he's trained in Mysore, India, travelled the world learning and teaching yoga, and been renamed. Kisen translates from Japanese as "Rare Transformation", which seems apt for a hard-drinking muscle man turned yoga teacher and Zen Buddhist. The bloke who used to think that yoga was "for pussies and weirdos" now says meditation is as routine as brushing his teeth.
Kisen's body shape has also been transformed. He used to weigh 13 stone, now he's down to nine. His waist measurement has gone from 33 inches to 28. Most amazingly he looks younger than he did 10 years ago.
He tells a story about a visit to his parents, after a gap of three years. His mum worried about his new name - "Do I have to call you that?" And his dad turned to him and said "I don't know what you're on son. But keep it up."
For more information on yoga classes contact The Life Centre (tel: 0171 221 4602) or The British Wheel of Yoga (tel: 01529 306 851). For information about Kisen or Zen Buddhist retreats contact Joanne Lakeland (tel: 01703 550968). Kisen will not be teaching at the retreat.