It is rare to be able to predict with any certainty the future of a restaurant, but in the case of the Petit Robert such soothsaying is justified. And so I can declare that it is a restaurant whose time has both come and will shortly go. The reason for it being of interest right now is its location in Borough, just south of the Thames in London. The reason for its eventual sad demise will also be its location in Borough.

Before we discuss the gloomy news, however, let's deal with the good stuff. At first glance the restaurant would seem blighted by that location. It is tucked away down the kind of cobbled lane on which children with rickets might once gleefully have danced. Down here, the only passing trade is probably the rodents scurrying bashfully down the dank gutters.

Walk just a dozen yards along the street, however, and you come face to face with the great, shadowed canopy of sooty old Borough market. Three years ago, a one-off food fair was held here for small, mainly organic producers of meat, fish, fruit and vegetables from across the country. It was such a success that, two years ago, the food market began running once a month. A year ago, it went to every Saturday, and now it is held on Fridays, too. Both days it throngs with food obsessives indulging themselves in a kind of ripe pornography for the middle classes. We move about its aisles, our lips moist with anticipation, ready to open our wallets wide in return for a particular kind of sensory gratification.

Here you will not only be confronted by the very best produce this country can offer, you will also find yourself at the heart of a network of suppliers which keeps the storerooms of Britain's best restaurants fully stocked. There's Turnips, the fruit and vegetable supplier, which sells to the Conran restaurants, to Aubergine and Nico Ladenis, and whose display is so plump and burgeoning it is practically indecent.

Next to them is Brindisa, importers of great Spanish charcuterie and cheeses, which supplies the Ivy in London, the Fat Duck at Bray and the Crooked Billet in Newton Longville, among others. Opposite is the stall for Northfields Farm, which supplies rare-breed beef to Hambleton Hall in Rutland. There's the Ginger Pig, whose stunning pork has been served up at Oliver Peyton's restaurants in Manchester, and further down the market there's wild beef from Hillhead Farm on Dartmoor, which is used at Gidleigh Park in Devon. The Wyndham's House Poultry Company from Yeovil in Somerset supplies the Conran restaurants; Seldom Seen Farm in Leicestershire supplies its organic geese to the Ritz and the House of Lords.

Naturally, now that the market is such a vast success, it is threatened with closure. Railtrack needs to expand its train routes into London and has decided that the only way to do so is to double the width of the viaduct that crosses over the top. A public inquiry into the development is underway, but many of the traders concede that the work is likely to go ahead in two years' time. And that will close down the market until 2004 and require the knocking down of a couple of dozen Georgian houses.

Though the weekly food extravaganza at Borough is a complete joy, it is still quite hard to justify going to the barricades for what is, essentially, an event for those with plump bank balances. (You can imagine the chant on the demo: 'What do we want? Wild beef. When do we want it? Just after the fish course.') But it's still a huge shame. Because when the work starts it will also take out the kitchen at the back of the Petit Robert and the restaurant will have to close.

So enjoy it while it's there. Like the market, it is a simple and straightforward place: a boxlike room painted a strangely calming shade of orange in which one can eat good-quality food in unpretentious surroundings. It should be said that the food is not the main reason for going there, though it is better in an unfussy French sort of a way, than that would suggest.

I went with another Robert, a restaurateur, who thought his starter of quail's-egg tart Florentine absolutely fine, if unstartling. While his partridge in a blueberry sauce could have arrived hotter, it was a good piece of gamey meat and a great, if shockingly blue, sauce. Likewise, my frogs' legs with garlic was the Ronseal of restaurant dishes: it did exactly what it said on the menu. Grilled rack of lamb was a sturdy piece of meat on a mustard sauce that was not too intrusive. Side vegetables were limp, but the chips were perfect.

So that's the food. The real selling point is the wine. Robert picked up the list and began muttering to himself. As far as he could see, the mark-up was rarely more than 100 per cent on retail, when the industry standard is around 300 per cent. The first bottle we wanted was out of stock so we chose another. Robert Didier, the Robert of Petit Robert who, as it happens, isn't at all (he's chunky and broad shouldered) was unimpressed. He suggested a Saint-Emillion which was £9 cheaper. It was an extraordinary bottle of wine for £20: deep and subtle. We asked how he could afford to do this. 'I don't bother with a percentage mark-up,' he said. 'I just make £5 clear profit on each bottle.' The result is some serious bargains.

We didn't fancy pudding, but a line at the bottom of the menu caught our attention. We were invited to turn the page for 'the best list of Armagnacs in London'. So we did. And it was. It began with an 1830 at what seems like a weirdly reasonable £165 a glass from the very last bottle in existence, and then moved on through the rest of the 19th century to cover most of the 20th. We each chose one from the year of our birth, a 1959 for Robert at £11 a glass and a 1966 for me at £6.50. They were both sublime.

The Petit Robert is a joy and, if you don't splurge on Armagnac, a pretty reasonable one (though, of course, not splurging on Armagnac would be missing the point). When the bulldozers come to Borough in two years' time it will, says not-very-petit Robert, relocate either to Devon or Oxford. That will be London's loss. But boy will some other town gain.

• Petit Robert, 3 Park Street, London SE1 (020 7357 7003). Lunch, including wine and service (but excluding Armagnac), costs around £60 for two. Borough market, off Borough High Street, London SE1 is open Fridays 12pm-6pm and all day Saturdays. Contact Jay Rayner on jay.rayner@observer.co.uk.