J Sheekey (the J is for Josef, the market stall-holder who in 1893 was graciously allowed by Lord Salisbury to sell fish and shellfish on his new manor of St Martin's Court, provided he served meals to Salisbury's theatre-going pals) is a legendary eating-house. I remember my mother pointing it out as a London landmark, along with Fuller's Cake Shop and Fortnum and Mason (this was the late Fifties, and food was clearly on her mind). I've always loved its solidity and class, even when it was getting down-at-heel in the 1980s. The mash carefully piped round the perimeter of your sea-bass was always the best mash. I once saw Anita Brookner lunching there with her agent, holding a cigarette in the same hand as her fork and taking puffs between mouthfuls: the epitome of ladylike decadence.

It started life as an oyster bar and the ancient premises expanded in November to accommodate a spanking new bivalve bistro – a gorgeous, Art Deco mini-palace, slightly crowded but wonderfully intimate, like the state room of an ocean-going liner. Smoky mirrors with orange sconces and monochrome photos of faded actors evoke a lost world of glamour; the reflective-mesh ceiling catches the lights from little table lamps and throws it over a vast bowl of exotic shellfish. Once you're settled at the new bar, you wonder if you'll ever leave.

The clientele is interesting in being comprised of two niche demographics: around the bar sits what seem to be a platoon of adulterous couples, expensively attired and tightly coiffed, while the banquette sofas are exclusively occupied by elderly ladies, possibly the original beneficiaries of Lord Salisbury's patronage. A trio of joshing Essex businessmen near the door seems quite out of place. The young couples eye up everyone who comes in; the old ladies watch the amorous transactions as though following a play. I've never known a restaurant where the punters checked each other out so much.

The food, though, the food. Naturally one starts with oysters. The least expensive are Strangford Lough Rocks which, with a glass of Roederer champagne, will set you back a mere £15 for 6, and very delicious the combination was: the oysters creamy and slithery, served with ceremony, crushed ice, and the usual spicy botanicals, and extremely filling. Hurrying past the Beluga/Sevruga selection (£250 and £125 for 50g) you find that the menu is eccentrically, though pleasingly, studded with cheap-ish starters and snacks in no particular order: goujons, crab bisque, gravlax, steamed mussels, even jellied eels are on offer alongside the posh lobster dishes.

My Cornish fish stew was a treat, seething with flavour, freighted with lumps of halibut, mussels on the shell, whole king prawns and sliced potato for extra ballast. My date's Atlantic prawn curry, served on a copper skillet, tasted fine – the prawn never overwhelmed by the cumin – but was boringly tepid. We sent it back and a replacement arrived after a long pause. It was an annoyance that brought the first wrinkle of anxiety to my brow. How could such a hot restaurant offer a lukewarm fish curry? This was Sheekey's, for heaven's sake.

By 2.15pm the place was almost deserted – inexplicably, unless the older patrons had gone for a little lie-down and the adulterers for an early-afternoon shag. It was good to have the Oyster Bar to ourselves, to spot John Sergeant heading for the cloakroom, to exchange badinage with Gordana, the maîtresse d' who used to adorn the Groucho Club, and to swig a glass of Sauvignon de Touraine. The ambience is so charming, we almost forgot about the food.

The pudding list is keen on nursery dishes (baked rice pudding, spotted Dick, rhubarb pie) which don't seem to fit the profile of a fish restaurant. My Scandanavian iced berries with hot white chocolate sauce were so cold my teeth ached; I couldn't quite see the rationale of serving frozen berries in March, but the effect is bracing. More temperature confusion beset the ruby plum and apple crumble with custard, which tasted fruity but was, again, feebly tepid. Crumbles should be piping hot. So should custard. They're rubbish lukewarm. "Why," I asked the chap who served us, "do you keep serving the food cold?" "It's a cooking error," he said. But it wasn't. It was a leaving-the-dishes-lying-around-for-half-an-hour-because-nobody-can-be-arsed-to-serve-them error. It was too late to ask for a replacement, so we had it taken off the bill (which still insultingly charged a discretionary 12.5 per cent service).

It was bitterly disappointing. Sheekey's Oyster Bar is a dream of a venue – chic, buzzing, convivial. The food, though the portions are small, is actually fine. You want to love it all. So how can they be so slapdash about temperatures? I came close to storming into the kitchen, upending the offending dish on the chef's head and shouting, "Don't worry – it can't possibly burn you. It isn't hot enough ..."

J Sheekey Oyster Bar, 28-32 St Martin’s Court, London WC2 (020-7240 2565)

Food 3 stars
Ambience 5 stars
Service 2 stars

Average price about £100 for two, including wine

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

Side Orders: World's your oyster

Riddle & Finns

This is Brighton's best oyster and seafood bar. Try 6 Rossmore Rocks (£10), washed down with a foaming tankard of black velvet.

12b Meeting House Lane, Brighton (01273 323008)

Loch Fyne Oyster Bar

This original Loch Fyne spawned a whole chain and has great views over the loch; try the oysters doused in chilli and coriander dressing.

Cairndow, Argyll (01499 600236)

Hix Oyster & Fish House

Helford Natives No 2s are £2.95 each at Mark Hix's sustainable seafood eatery overlooking the Jurassic Coastline in Dorset.

Cobb Road, Lyme Regis, Dorset (01297 446 910)