Northbank, Millennium Bridge, One Pauls Walk, London EC4, 020 7329 9299

Riverside-view restaurants aren't always a boon to mankind. Dining at the Imperial in Bangkok is like an exotic dream, as you watch the lantern-hung bateaux mouches criss-cross the Chao Phraya river like huge fireflies. Dining by the harbour in New Orleans is a less delightful experience, watching tramp steamers wheeze and clank up the Mississippi. When it comes to London, there are some eateries whose windows frame the electric wonderland of the City, but it would take a brave man to try to persuade us that the south bank of the river is a gorgeous sight.

Christian Butler, late of the Adam Street club off the Strand and Baltic in Waterloo, is that man. He's reinvented an old restaurant by the Millennium Bridge, taking it soaring upmarket; its unique selling point is that diners can gaze across the slimy, wolf-grey river, drinking in the sight of Tate Modern. Is that enough of a treat to bring people flocking? Luckily, his establishment has a second USP: it features almost exclusively ingredients and dishes from south-west England. The result is an almost unmitigated triumph.

The bar is small and friendly and serves a wicked fino sherry. The restaurant is full of light and if you're lucky you'll get one of the banquette-seating booths, in which you sit surrounded by mirrors and wallpaper featuring scenes of al fresco misbehaviour. My friend Sarah and I sighed over the menu; it's one of the rare sort, on which you feel you could eat practically everything. Could you resist hog's pudding with honey roasted apples? Of course not (it's a mild version of black pudding). Or Falmouth crab with clotted cream tart?

We fell upon two familiar dishes that had been given a surprise twist. Steamed mussels with cider and smoked bacon offered a butch alternative to the usual marinire treatment, and the blue tureen in which they came disgorged a wonderful flavour. The fat bivalves were succulent, the bacon-shallots-and-cider soup a brilliant invention. Sarah's braised oxtail ravioli with creamed leeks and truffle was "miraculous exactly what ravioli should have in it, something very strong-tasting. It's very masculine, but little ladies could gorge on it happily." The home-made pasta melted in the mouth, while the leeks added an onion-y asperity to the earthy flavours of ox and tuber. As starters go, it was an utter smasheroo.

I began to feel proud of the south-west, for coming up with stuff like this. I liked the way the menu promised: "Some of our dishes may contain nuts or shot". I liked the way both owner and general manager happily sprang into the roles of barman or waiter if they saw any luncher look around for help. Through the window, the long pterodactyl ribs of the Millennium Bridge gleamed in the unseasonal sunshine. I started to like the Northbank immensely.

The starters were a tough act to follow and, truth to tell, the mains offered more prosaic delights: rib-eye steak and chips, lamb stew with herb dumplings, Dover sole with caper and shallot butter. Sarah chose roasted pheasant breast with braised red cabbage, which came served on a bed of creamy mash, arrayed with chestnuts. "There's a lot of things fighting for the upper hand here," she remarked, "but the pheasant itself is tender and moist, if not as miraculous as the oxtail."

I ordered spiced "West Country" pork belly with clams and smoked sausage, mainly out of interest: pork belly with clams? When did pigs and seafood co-habit in a pan? It wasn't a triumph: the fat had been removed and the belly rolled up into a tight block and roasted a little over-enthusiastically. It was a touch dry, the accompanying spicy sausage knocked it for six and the four attendant clams, in their pale shells, stood around uncertainly like tiny gatecrashers at this porky feast, adding nothing to it. I asked the manager: was this really a West Country dish? "Pork and clams is actually a Catalan recipe, but we thought we'd try it with Cornish razor clams," he said. "We haven't been able to get the right quality, though, so ..."

So they used ordinary clams instead? It was the only bum note in the whole meal. Later, as I tucked into a big ramekin of pear and blackberry crumble with custard, and Sarah ingested forkfuls of date and vanilla tart, the waiter brought two, on-the-house, slivers of Epoisses cheese, because he'd heard me enthusing about this prince of fromages. They melted creamily across the plate even as I looked at them. With charm like this on tap, you'd forgive them anything, wouldn't you? A Lebanese wine called Massaya (17 for a 500cl carafe) proved a perfect companion, redolent of leather and smoke. The excellent coffee, and a slug of Pomona Somerset cider liqueur, rounded off a near-perfect lunch. We left feeling replete and, by now, half in love with the view of Tate Modern. Northbank is a delightful addition to the London restaurant scene, and I'll be back as soon as possible.

Northbank, Millennium Bridge, One Pauls Walk, London EC4 (020-7329 9299)

Around 100 for two with wine