The cyclist talks about his voyage from doper to anti-doping crusader and the state of cycling as the Tour de France kicks off.

It is 1.30pm and Scott's of Mayfair, a renowned fish restaurant on one of London's smartest streets, is buzzing. White-aproned French waiters dance between tables at which expensively dressed men and women are toasting their latest successes. At one end of the small room former snooker world champion Ronnie O'Sullivan is entertaining a group of friends; at the next table society designer Nicky Haslam makes a glamorous journalist laugh, and to my right some executives from Louis Vuitton are engaged in deep conversation with a prominent magazine editor. My table, however, is silent.

David Millar is late and, as I sit re-reading the menu and examining the cutlery, I start to worry whether he's coming at all. After all, professional cyclists are not known for their hearty appetites, especially in the run-up to the Tour de France, the biggest race of the year, which starts this weekend. Fans are familiar with whippet-thin figures hunched over their bikes and articles in cycling magazines describe obsessive regimes to reduce body fat, and thus avoid carrying unnecessary weight up the Tour's vicious mountain climbs. But Scott's menu features oysters with wild boar sausages, fresh Devon crabs and lobster thermidor: if Millar does turn up, will he insist on a protein shake and stick of celery?

My fears are unnecessary. When Millar rushes in, 30 minutes late, he apologises, blames the traffic, orders a beer and starts discussing the menu. "I've been looking forward to this all day," he says, eyes gleaming. "I love restaurants like this, that classic French service, all so businesslike." I gingerly push the wine list across the table. We are meeting three weeks ahead of the Tour and I can't help feeling that I might be leading him astray. But Millar doesn't demur, and orders a bottle of Viognier Sainte-Fleur 2008.

In fact, being led astray is a large part of what people know about Millar. In 2004, he was reigning world time-trial champion, leader of French team Cofidis, with a string of race wins to his name, a million-euro annual contract and a playboy lifestyle. And then, on a summer's evening in one of Biarritz's best restaurants, a team of policemen burst in, grabbed Millar and bundled him into a police van. After two days in a cell, he confessed: he had repeatedly used performance-enhancing drugs. "I knew I was going to lose everything – the house, the car, the lifestyle, the job, the respect ..."

I suggest we leave the drugs until we have ordered – seared scallops with garlic butter for him, smoked salmon for me, followed by cod with Padrón peppers and chorizo for both of us – and ask him what it was that first attracted him to cycling. He says he took up mountain biking in his teens, having gone to live with his airline-pilot father in Hong Kong, but then, aged 15, road cycling began to fire his imagination.

"I started learning about the sport, reading about it, and I was just enchanted," he says. "It seemed romantic but also tragic – people would be winning but then lose it all, or crash but fight on, break bones but get back on their bikes and try to finish. Just getting to the end was seen as an achievement in itself. It's somehow old-fashioned, gladiatorial ... "

Masochistic? "Absolutely – it's all about suffering. Often the best guys are just those that can suffer longer, who don't give up. And it's so easy to give up, when you're on a mountain and it's really hurting. We go through a lot physically."

And, so ... the attraction? "Well, they say it's like hitting yourself in the head with a hammer – when you stop it feels great."

He describes a race in Switzerland in which 200 riders started but only 15 finished: "The course went up a mountain, then down the other side to the finish, and it started raining, then snowing. On the descent it was so cold my fingers couldn't work the brakes and guys were crashing off at all the corners. When I crossed the line, I was hypothermic and started having full body convulsions." I nod sympathetically, my mouth full of rich smoked salmon.

Millar, 34, is wearing a charcoal-grey Paul Smith jacket, funky thick-rimmed glasses, slicked back hair and a deep tan. When he began to become known in cycling, French newspapers nicknamed him "le Dandy" – "I hated that!" he protests – but he still looks more like someone who works in graphic design, fashion or film. The tan comes from living and training in Girona, Spain, but he is in London for the launch of his autobiography Racing Through the Dark, the pages of which drip with visceral descriptions of the agonies of cycling.