Star grazing: The wonderful world of Giorgio

When Brad Pitt wanted to entertain Madonna, Guy, Patsy and Jamie last week, only one venue would do: Locanda Locatelli. Hermione Eyre on the restaurant that upstaged The Ivy to become the canteen to the stars

It was an evening like any other at London's Michelin-starred Locanda Locatelli. A cosy little table of five were settling in for dinner. "Pass us a Parmigiano crostini wotsit, will you love?" said Patsy Kensit, nudging Brad Pitt. But Brad was too busy listening to Jamie Oliver explaining what lampascioni are (pickled onions). Madonna, for her part, was athletically pursuing her napkin, which was missing under the banquette, so it fell to Guy Ritchie to pass the bread basket over to Patsy's side of the table. Meanwhile, conversation turned to the merits of regular lymph massage ...

Who knows what the A-list table that assembled at Locatelli last Tuesday really talked about. That secret is held by the restaurant's very discreet staff. (Heck, they even sign a confidentiality agreement as part of their contract.) But what we do know is that Locatelli has roughly the same effect on celebrities as a magnet has on metal filings. Sven dines Nancy there (or should that be the other way round?), Bill Clinton and Simon Cowell eat here (though not together), and Lucian Freud brought Kate Moss here during a break when he was painting her portrait. As Plaxy Locatelli (wife and business partner of founder chef, Giorgio) puts it, "It's more a case of who hasn't been here."

But why is it such a hit? The food is superlative, but there are 36 other restaurants in London that also have Michelin stars. Giorgio Locatelli, who opened the restaurant in February 2002, is a tousled, energetic presence in the restaurant, but plenty of other chefs have charisma too. The reason celebrities flock here is, paradoxically, because it doesn't look or feel like a celebrity haunt: it is an ultra-discreet hideaway. As one regular told me, "There are never paparazzi or a fuss. It's smart and stylish, but it doesn't feel like a big deal." Indeed, Locatelli's is reassuringly hard to find.

Tucked away in the ground floor of a stuffy hotel (The Churchill on Portman Square), Locatelli's entrance is all frosted glass and thick, concealing curtains. Inside, the lighting is flatteringly low and the décor a symphony of blond wood and taupe - the kind of background, in fact, against which a TV frontperson with a tan and expensive highlights might disappear entirely, if they sat very still. It's a celebs' natural habitat. And compared with the fishbowl of The Ivy ("where you're either a star or a rubbernecker," according to one regular) and the aggressively trendy Nobu (where paparazzi pitch permanent camp on the doorstep), Locanda Locatelli is a private haven.

This takes work, of course. Roberto Veneruzzo, the maître d', confides: "If we see there are photographers outside, we help our guests find a different way out of the building." Veneruzzo is venerated by his regulars: Simon Kelner, the editor of The Independent, calls him "the best maître d' in the world".

Now Roberto has a new phenomenon to keep in check. "Guests have been taking out their phones and doing - click!" Camera phones are clearly the 21st-century autograph book. "We ask them, do you mind, er, not doing this..." But Roberto is forgiving. "You know what? We understand. They are young... It is Brad Pitt..."

In fact, Plaxy also understands very well. She wouldn't have minded being in the restaurant herself when Mr Pitt came to dine. "I was at home looking after our daughter. When Giorgio told me, I was like, oh no!" (Giorgio, too, takes an endearing pride in feeding famous names. He once said, rather suggestively, "It's nice when you cook something and you see Liz Hurley swallowing what you done.") But Plaxy is set on Brad. "He is a really special case. I understand he caused a bit of a stir among the other diners. Women who were there having loving dinners with their partners started swapping seats to get a better view of him ..."

Ah, the view. A swanky restaurant works rather like a theatre: some seats are better than others. At Locatelli's there is the Royal Box (central, elevated) where Madonna sits in state surrounded by her family. There are good stalls seats (bigger central tables) where Gwyneth Paltrow, Chris Martin and Helena Christensen are to be seen, all happily eating macrobiotic, no doubt, since the restaurant is very accommodating of complex diets (the Locatellis' daughter suffers severe anaphylactic allergies, so they are well practised).

Then there are the house seats, table 11. "We call it the chef's table," says Plaxy. "We hold it back for regulars or friends or family. It's in a very quiet table by the kitchen doors, so it's where you sit if you are famous and you don't want to be overheard or if you are proposing - or divorcing." This discreet spot, then, is where we will imaginatively place a courting Woody Allen and Soon-Yi, holding hands under the linen tablecloth. Finally, there are the cheap seats. At Locatelli's, these are unmistakably the far seats on the right by the entrance. There's a draft of Siberia about them. You might seat visitors Tania Bryer here, or possibly Cecil Parkinson.

For all the celebrity fuss, though, Giorgio Locatelli has one customer he would like to thank. That's a lawyer called David Silver who, in 2002, refused to pay for a truffle-themed starter he had ordered (and consumed, no doubt, with relish) before he realised it cost £30. Giorgio held the man's girlfriend "hostage" until they paid up; the story created a media frenzy. "It's funny," he says, "but we were all sitting here, voting for who we thought was our best customer, and everybody was saying Madonna and Eric Clapton, but I say it's got to be Mr Silver because he came, he ate - and he gave us a million pounds' worth of free exposure. After the truffle story came out, our sales of truffles went up by 75 per cent!"

Spoken like a true realist and someone who knows the value of deflecting attention from the stars. Serving them has made him famous but it's discretion that keeps them coming back.