It has a mythical appeal as one of the world's great clarets and costs at least pounds 200 a bottle. Now it is the star of a new movie. So just what's so special about Cheval Blanc?

All along the empty, pretty lanes of Saint-Emilion, small, claret- coloured signs point the way to Chateau This and Chateau That. One chateau name is missing. No one has bothered to erect a sign to Chateau Cheval Blanc. Does an Oscar-winning actress put her number in the phone book? Does royalty advertise? You know you have arrived when you see the modest, carved sign at the entrance. Between the public road and the first of the Cheval Blanc vines, standing in gnarled rows like hunched old ladies at this time of year, there is a shallow ditch. That is all.

There is no gate; no fence; no hint that you are in the presence of greatness; no clue that you are standing beside some of the most magical soil in the world. There are, encouragingly for minimalist gardeners everywhere, masses of weeds between the vines. (There is a good, ecological reason for the weeds; explanation later.) You might be looking at a field of turnips, not one of the most consistently marvellous, and most prized, of all red wine vineyards.

Some experts, including the controversial American wine critic Robert Parker, may press the case of two or three other Bordeaux domains, including those of the greatest of Pomerols, Chateau Petrus, visible as a smudge on the misty horizon a kilometre to the west. Others may point out that the most absurdly expensive of all red wines comes from the minuscule Romanee Conti vineyard in Burgundy.

To the hero of the hit film Sideways - a middle-aged Californian wine- obsessive, who, as far as we know, has never been to France - there is only one wine worthy of being used to seduce a woman. Miles, the hero of the movie, regards Chateau Cheval Blanc and especially the "legendary" Cheval Blanc 1961, as the holy grail of wines.

Why Cheval Blanc? Until now, it has been a discreet wine, a connoisseur's wine, always admired but less publicly feted than the great Bordeaux media stars, such as Petrus or Chateau Latour. Mr Parker, of the American Wine Advocate, the boyish wine guru from Maryland who has a hypnotic influence over American tastes, is no particular fan of Cheval Blanc. Even its mythical 1961 vintage scores "only" 93 out of 100 in Mr Parker's scoring system. The relative obscurity of Cheval Blanc to the grand public is probably about to change. The small domain on the borders of Saint Emilion and Pomerol should brace itself for an invasion of rubber-neckers and gawpers this summer. I was the first of them; but I was expected. There is only one industry in France where you could ring up in the morning and be invited to come to see the boss in the afternoon. The French wine business, even in its rarefied upper slopes, is astonishingly welcoming to outsiders. (Please note that there are no public tours at Cheval Blanc and no wine sales. You cannot roll up with a car full of kids and buy a crate of Cheval Blanc 1995 at, say, Û400 (pounds 275) a bottle.)

The manager of Chateau Cheval Blanc is Pierre Lurton, 48, member of a great Bordeaux wine-growing family. Like many of the leading wine people in the "Bordelais", you might mistake him for a British aristocrat or a bouncy, off-duty British Army officer: sports jacket and waist-coat, open-necked shirt and a thatch of curly, sandy hair.

M. Lurton has not seen Sideways. "No time, Much too busy." He has read an account of the plot. He is amused but not surprised that his wine should have become a silent star in an Oscar-nominated, Hollywood movie. "For me, it is quite plausible that the hero should be fixated with Cheval Blanc. It is a mysterious wine, an elusive wine, a powerful, an elegant wine but never an aggressive wine, one that comes to obsess people, because it defies all the normal Bordeaux categories. Cheval Blanc is a Saint- Emilion but is not really a Saint-Emilion. Although we are next to the frontier with Pomerol, it is not a Pomerol. Cheval Blanc is like no other wine. It is Cheval Blanc."

Cheval Blanc is unique because it stands on a 37-hectare (90-acre) "mosaic" of sand, gravel and clay which replicates part of the soil and climatic conditions, the terroir, of Saint-Emilion and partly those of Pomerol next door. It is unique because these conditions allow the chateau to grow a mixture of merlot grapes which love clay (about 40 per cent of the total) and cabernet franc grapes which prefer gravel and sand (about 60 per cent).

Because the cabernet grapes are less productive than the merlot, the final mix in Chateau Blanc is usually 50-50 between the two grape varieties. No other wine in Bordeaux has such a high proportion of cabernet franc, which has a reputation as a difficult and complex grape. (Cabernet sauvignon is the more common partner for merlot in the Bordelais.)

The combination might not work elsewhere but at Cheval Blanc, for some reason, it works triumphantly: the rich, fresh youthful taste of the merlot is made more complex and longer-lasting by the "elegance" of cabernet franc. Chateau Petrus, across the fields, uses 97 per cent merlot in its famous wine. Excellent though it is, M. Lurton hints he regards it as a mere fugue, played on one theme, compared to the orchestral complexity of Cheval Blanc, playing on the different qualities and different growing patterns of the two kinds of grapes.

One of the oenological blunders in Sideways - perhaps a deliberate blunder to amuse the wine lovers in the audience - is that Miles, the wine bore, rails against the merlot and cabernet franc grapes. At the same time, he idolises Cheval Blanc, which is a half-and-half blend of the two. His other obsession is the pinot noir grape, which is shunned in Bordeaux but produces all the fine red wines of Burgundy and is one of the main components in champagne.

Some Bordeaux wine experts also question his obsession with the 1961 vintage. "Only an ignorant Californian would make a fetish of the '61," said Nicholas Faith, one of the foremost British writers on Bordeaux. "The real legend is the '47. Now there was a wine to die for." M. Lurton, who has been manager of Cheval Blanc for 14 years, has a few bottles of both vintages in the ancestral cellars, for special occasions but definitely not for sale. He says that both are sublime. "In the Bordelais, 1961 was a year of frosts, which reduced the yield of grapes but concentrated their strength, giving a very powerful, very profound wine."

You can still drink the 1947 which is a tribute to Cheval Blanc's staying power. Compared with other great clarets, it is also a wine which can be drunk relatively young, after maybe seven or eight years. Ideally, you should wait until it is 15 to 20 years old.

The speculative boom in the price of the best clarets in the 1990s, now somewhat abated, has pushed Cheval Blanc out of the reach of ordinary mortals. At auction, you could expect to pay up to pounds 3,700 for a bottle of Cheval Blanc 1947 (100 out of 100 in the Parker scoring system). The 1961 vintage goes for a relatively modest pounds 750 to pounds 950. Even for a bottle of 2004 Chateau Blanc - to lay down in your own ancestral cellars - you could expect to pay about Û220 (pounds 140).

Cheval Blanc was founded in 1832 and has been regarded as one of the top seven or eight Bordeaux chateaux for more than half a century. It was the 1947 vintage which made its reputation. "Other domains have had rocky periods," Mr Faith said. "Cheval Blanc has always been wonderful."

The name is something of a mystery, and a wonderful snub to marketing men and women everywhere. Which PR expert would allow an expensive red wine to be called "white horse"? For more than a century, Cheval Blanc was owned by the Fourcaud-Laussac family. In 1998, after a family quarrel, it was sold jointly to Baron Albert Frere, a Belgian industrialist, and to Bernard Arnault, the founder of LVMH, the world's largest luxury goods group.

The men are friends but make an odd couple. One Bordeaux insider said: "When Francois Pinault [the great rival of M. Arnault as a French self- made billionaire], bought Chateau Latour, Bernard Arnault had to have a leading chateau of his own. He knows nothing about wine, but luckily he does not interfere. Baron Frere reputedly drinks only water but he knows a great deal about wine. He is on the telephone every day."

A great and poisonous debate in the wine world - the debate dramatised in another recent wine film, Mondovino - opposes "wine-making" to terroir. Wine-making, at its worst, is an attempt to impose a commercial and uniform taste on a wine. Many Bordeaux chateaux, including some of the great names, are accused of using itinerant wine-makers, such as Michel Rolland, the great villain of Mondovino, to "Parkerise" their wines. To ensure high scores in the Parker system, they stamp on to the wine the powerful, rich, heavily oaked taste Mr Parker appears to prefer and blot out or dilute its typicite, or special characteristics.

This is sacrilegious to the eyes, and mouths and noses, of French (and not just French) traditionalists, who believe that the soul of a wine lies in its terroir, its soil and micro-climate, rather than in the wine- making process.

M. Lurton at Chateau Cheval-Blanc is definitely a terroir man. "Everything is in the grapes. If you get the grapes right, there is no particular mystery in wine-making," he said. He regards himself as a "guardian of the temple", someone whose duty is to ensure the grapes grow in the best possible conditions, then to interfere as little as possible. For this reason, Chateau Cheval Blanc is as close to organic as you will find in a great vineyard. Herbicides are banned (hence the flush of weeds which will be ploughed under in the spring). Insecticides are used very sparingly. Artificial chemical fertilisers are outlawed.

The other great debate in the French wine world - the future of the lower- market and middle-market French wines, undermined by New World and Italian competition - hardly concerns Cheval Blanc. The aristocrats among French wines have never been so aristocratic and the bourgeoisie and sans-culottes of wines never so miserable. The wholesale, barrel price of generic red Bordeaux has fallen to just more than Û1 a litre this year, in other words, 200 times less than a bottle of Cheval Blanc.

Should the French middle-market wines persist in the terroir and appellation controlee system? Or do they copy the New World opposition and market themselves by grape varieties - chardonnay, merlot etc - with a smaller number of cheerful, easily recognised, supermarket-friendly labels? The debate will go on but the switch to grape variety French wines in the middle market is already happening.

M. Lurton, although a great believer in "terroir", says the case for great "visibility" and "simplicity" in the marketing of middle-range French wines is unanswerable. He is right, but it is still a pity. "Terroir" wines may gradually become the preserve of the rich or the wine-obsessed, such as Miles in Sideways.

How can anyone justify a Û200 bottle of wine, I ask? And how should a wine like Cheval Blanc be drunk? M. Lurton throws his arms up in the air, partly agreeing, partly disagreeing. "Cheval Blanc is best drunk among a group of friends," he says. "Preferably when someone else is paying. It should accompany a special moment, a moment that will always be remembered and the wine will help to mark that moment, to engrave it on your memory. Cheval Blanc is expensive because it is a great rarity and a great luxury. Like all luxuries, it demands a high price."

M. Lurton then invites me to try a couple of recent bottles of Cheval Blanc, a 1999 and then a 2001. Both are too young for normal drinking but they are already extraordinary, especially the 1999. There is a kaleidoscope of taste, stretching into the far distance. It is as if you had glimpsed the Mona Lisa when she was still just a sketch in Leonardo's notepad. But what a sketch.