Some excellent restaurants are like secrets, tucked away from brashness and bustle, apparently known only to a select crowd of zealots and devotees. Roussillon is a classic case. Everything about it breathes veiled discretion. The Pimlico street in which you eventually gaze at its bow-windowed loveliness is so discreet, it doesn't even show up on my online Streetmap. Ring for directions, and you're told to park the car and walk.

Inside, the décor is so understated, so determined not to strike any discordant note, that I failed to take down any details about it. I could have been in a sensory-deprivation tank. I remember the floor was covered in plush brown carpet, in a way few restaurant floors are these days, the tables are spaced far apart so as not to catch your neighbours' conversation, and the lights are shyly recessed into the ceiling. The whole atmosphere is hushed and whispery and intriguingly foreign. At the neighbouring table, a handsome, silver-haired Mediterranean murmurously romanced a slender jeune fille. On my left, two German chaps straight from a Gillette commercial talked mezzanine financing. This place could be the perfect haunt of well-heeled spies exchanging state secrets, or a couple of chaps from the IMF very quietly discussing what exactly happened...

The £65-a-head menu is laid out in a stridently non-secretive way. Down the page, the dishes are announced in shouty bold capitals, with starters from LOBSTER to FOIE GRAS, and mains from TURBOT to DUCK. The effect is curiously as though you're being addressed by MICHAEL CAINE. But at least it's perfectly clear what's on offer. What is less clear is why we need so many amuse bouches: cream cheese on a soft biscuit, a wafer of Parma ham on a cube of melon, and an "essence of vegetable ratatouille" in a shot glass, one of the more off-putting amuses I've been offered. But while the first two were ordinary, the "essence" was lovely, like gazpacho consommé. Whether we needed a further mouth-tease – of fresh-water prawns in a pea veloute with a garnish of broad bean flowers – wasn't clear. It meant that, before the hors d'oeuvre had arrived, my taste buds were rushing about in confusion shouting "Enough amusement!".

What followed, though, was exceptional. The head chef, Dan Gill, is ridiculously young (24) but is a protégé of Raymond Blanc at the Manoir aux Quat' Saisons, and has found his own style at Midsomer House in Cambridge. His cooking is astonishingly complex (hearing him describe his methods is like watching Einstein inscribe fractions on a blackboard) but always in the service of vividly connective flavours. My date's lobster salad with mango, avocado and chorizo may sound like simple food combining but it's not: the lobster was killed to order and served at room temperature, avocado came in purée form and au naturel, the mango was both sliced and sauced, the chorizo glowed in 20p slices rather than in ignorant cubes from a supermarket sausage. The ingredients all embraced each other in an orange-hued, subtly-spiced group hug. My pan-seared scallops offered the usual challenge of finding some gutsy accompaniment to their plump and juicy innocence: Gill's solution was a fabulously tasty rectangle of ham hock that had been boiled for hours, shredded, pressed overnight with capers, gherkins, parsley and tarragon, rolled in breadcrumbs and lightly fried. All this labour paid off – it ran rings of savoury gorgeousness around the scallops, to be finally anointed with an onion and cardamom sauce.

Angie's main-course turbot was less triumphant, despite its having been rolled in a hazelnut crust – so many nuts that it seemed a desperate attempt to zhuzh up the bland turbot. The carrot and saffron sauce only added more blandness. But the masterstroke of leaving a helping of crab (or "cock crab" as the menu quaintly puts it) at the bottom of the dish saved the day. My roast loin and crispy belly of Lancashire suckling pig came as three slender loin chops on the bone, perfectly roasted, their skin turned to nicely dry crackling, a gleaming ochre bracelet of pork belly that was too fatty for comfort, and a cockle and fennel remoulade that contrived to be both sensuous and wonderfully crispy. An accompanying langoustine, simply cooked, made no logical sense (surf and sty? Is this a new thing?) but tasted fine.

A pudding of strawberry flavours threw together every condition of the fruit you could think of – ice-cream, jam, jelly, wild, cooked, tuile – and, while lovely, lacked sophistication; I felt about eight years old while eating it, but was comforted by the best pudding wine in town, a 2002 Hetfurtos Tokaji, chosen by the charming Hungarian sommelier, Gerjei.

In every other respect, the Roussillon is a sophisticated delight that should cease being a secret.

Roussillon, 16 St Barnabas Street, London SW1W 8PB (020-7730 5550)

Food 4 stars
Ambience 3 stars
Service 4 stars

About £200 for two, with wine

Tipping policy: "Service charge is 12.5 per cent discretionary, of which 100 per cent goes to the staff; all tips go to the staff"

Side Orders: Chelsea charmers

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