You won't find every wine in La Tour d'Argent's cellars, but if it's a £7,000 corker you're thirsting for, there's nowhere like it. Alix Sharkey goes underground in Paris's oldest - and most celebrated - restaurant, where France's finest vintages are kept under the nose of one very proud Englishman

La Tour d'Argent is a Paris institution. For a start, it bills itself as the city's oldest restaurant, an inn of the same name having been established on the site in 1582. Though comparatively modern, its menu has altered little over the past century, during which time its sixth-floor dining-room, with sweeping views over the Seine and across the city's rooftops, has attracted countless heads of state, royals, artists and stars of sport and cinema.

Its foyer walls are lined with autographs of celebrity diners including Churchill, Clinton, Chirac and any number of Hollywood A-listers. A monochrome photo shows a young Queen Elizabeth drawing up her gown as she leaves. Pele's signature hovers above Ronaldo's; an original Jean Cocteau drawing of a former maître d' nestles alongside David Lynch's glowing testimonial.

Typically, La Tour is a family concern: current owner Claude Terrail, now in his eighties, inherited it from his father André, and will be succeeded by his son, also André. The real-estate value alone would make the property one of the city's most coveted. Yet the real family jewels are actually hidden from view, some 15 metres underground. The wine list gives a clue - a hefty, silver-embossed leather tome, it includes some 11,000 vintages.

For the labyrinthine cellars of La Tour d'Argent are undoubtedly the best-stocked in France, and perhaps anywhere. Stacked floor to ceiling in a warren of dark, cool, musty alleys, are nearly half a million bottles of the world's rarest - and costliest - vintages. These, to paraphrase Richard E Smith's character in Withnail & I, are "the finest wines available to humanity". And every last drop - except for the odd bottle of port or sherry - is French.

Amazingly then, the chef sommelier responsible for maintaining the quality and abundance of this liquid treasure trove is from Woking, Surrey. Before coming to Paris at the age of 20, 42-year-old David Ridgway trained with the Roux brothers in London. Having started at La Tour as a bottle-washer, he progressed "the normal way - you arrive at the right time, work hard, and push everybody else out of the way".

But isn't it unusual - even unthinkable - for one of France's most prestigious restaurants to have an English chef sommelier? Hadn't his appointment aroused French cultural chauvinism? "Well, it wasn't particularly something we advertised at first," says Ridgway, who looks like Guy Ritchie's more hearty and sociable elder brother. "But nowadays - I've been here over 20 years - I suppose it's part of the tradition. And after all, the English were once regarded as connoisseurs of French wine."

Refreshingly, Ridgway is the antithesis of the clichéd wine snob you might expect in this post. Though his cellar is exclusively French, he enjoys New World wines, too. "I like drinking, and it's fun to try all kinds of wine. Sometimes the complex, so-called 'intellectual' wines are a bit boring. And if you only drank expensive French wines, I think that would be rather sad. You'd forget what other wines taste like, and then there'd be no point in drinking the good stuff."

Anyway, says Ridgway, even venerated vintages can be disappointing. "It's like meeting your favourite film star. Sometimes they don't live up to your idea of them."

La Tour's cellar specialises in fine Burgundies, Ridgway tells me, because these are the hardest wines to buy, "and that makes it more fun". Burgundy is France's oldest wine-growing region, he explains, and its vineyards are small, family-run operations * which produce tiny amounts of many different wines, rather than lots of a single wine, as with the larger growers in Bordeaux.

"Many of the best Burgundy wines are only made two barrels at a time. That's 600 bottles for the whole world. And most of those will go to wealthy private clients. So you've got to get in early, you've got to know the guy personally - otherwise he's not going to sell it to you - and you've got to know which years are the best."

Aside from visiting vineyards and maintaining les caves, Ridgway works on the restaurant floor with his team of 15 sommeliers, directing customers to the wine best suited to their food, taste and budget. It's sometimes a fine call. In the old days you could judge a customer by their dress, in particular by ties, medals, jewellery and shoes. But today's dotcom millionaires and media moguls arrive in basketball caps and stealth-wealth civvies which, though often expensive, look at first glance as if they are from Gap.

"You need a fair amount of tact and diplomacy," says Ridgway, "because some ultra-rich people will take offence if you suggest a wine that's not expensive enough. They see it as something of a slight. Anyway, the idea is to advise, not to impose. But I suppose we are like doctors - we always think we know best."

Of course, La Tour d'Argent's legendary cellar has its share of wines that attract "a certain type of customer", like the vintage Romanée Contie, a rare and exceptionally fine Burgundy. Don't bother looking for it down at your local Oddbins - only 1,800 bottles are produced each year. La Tour d'Argent sells roughly 10 a year, at just under £7,000 a pop.

This, though, doesn't even figure among the most expensive wines, which are kept in a double-locked strongroom by Ridgway's office. It requires two keys - his and another staff member's - to open the door. "It's so I can sleep at night," says Ridgway, who estimates the room contains "a couple of million pounds' worth" of wine and champagne - including vintages of all the Bordeaux first growths, the "noble" wines like Lafitte, Petrus and Mouton-Rothschild.

Many of these arrived in 1914, when André Terrail married into the family which had owned the prestigious Café Anglais. Though demolished in the 19th century, during Haussmann's remodelling of Paris, the contents of that restaurant's legendary caves had remained intact, and were presented to Terrail as a kind of dowry. A quarter of a century later, they were saved once again, when Claude Terrail built a false wall to hide the best bottles just hours before the Nazis took Paris during the Second World War.

Much to the frustration of the many wealthy customers who ask to buy rare vintages to take home, the restaurant refuses to sell unopened bottles. Some take offence, others simply return to drink their favourites. Occasionally though, an individual will not be deterred.

In the 1940s, billionaire banker Pierpont Morgan asked to buy one of La Tour's two bottles of extremely rare Fine Napoléon cognac, and was politely refused. Some months later, three of his staff dined at the restaurant, and asked for a tour of the cellars. After they left, Ridgway's predecessor found one of the two bottles missing, and in its place a blank cheque, signed by Morgan. To show his utter contempt, André Terrail returned the cheque, uncashed.

"It was a wonderful gesture," says David Ridgway. "But if somebody pulled the same trick today, I have a feeling the cheque would be banked, and with a lot of noughts on it."

La Tour d'Argent is at 15 quai de la Tournelle, Paris, tel: 00 33 1 43 54 23 31,