The Mansion is a fantastic sight on the road from Dulwich to Crystal Palace. It's a great bulky lump of Victorian architecture, as solid as a fortress, and dominates the landscape. Once it was merely a boozer called The Paxton but, since it was bought by Ben Sowton (the guy responsible for the terrifically groovy White House in Clapham), it's been transformed. The ground floor's painted black outside, the first floor's all brickwork with black trim.

Inside, it's not so much a pub as a lounging area, with brown leather sofas, newspapers, a grand piano and a bar attached, almost incidentally. The décor is antique-shop chic: you can hardly move for ornate mirrors, elaborate chandeliers and the kind of square tables you'd find in a correctional facility. Huge, bald south-London men shoulder their way to and fro, but it feels friendly: the manager bustles about in blue jeans, the Aussie waitress is charming, a couple are enjoying a little snog beside the bar. It's a place for sing-songs, trivia quizzes and Friday-night flirting.

The eating area extends around the bar, down a slightly dismal corridor and into a separate dining-room beside the kitchen. Tonight, a private party is making whoopee in there, so we're sent to dine in the bar. In a south-London pub. On a Saturday night. In November. This is not, it turns out, a brilliant idea.

The Mansion is hovering in that tricky transit lounge between being a pub that serves food and a gastropub that chases awards. Its menu goes well beyond bar snacks, but plays it safe and predictable with English meat, fish and cheese. Dinner began with a flourish, as half a loaf of delicious crusty bread came to the table (though my date, Carolyn, complained that it was steamed and therefore grossly unacceptable). Her pressed ham hock terrine was fine, if a little oversalted, and the accompanying lentils had been cooked in a proper stock. My deep-fried whitebait, spiced with cayenne pepper, were blanched to an unearthly whiteness but, sadly, weren't as crispy as whitebait must be: I suspect the oil in which they were deep-fried just wasn't hot enough. When drenched in mayonnaise, they tasted fine – for a pub. But this place has aspirations to be more than that.

By 9.15pm, the place was getting noisy. Some of the chilled-out throng around the leather sofas began yelling things at each other, in a kind of chav-antiphon. I asked the waiter where the produce came from. "Locally," he said. "More precisely, where from?" I asked. "Oh, you know – Kent, Berkshire," he said. "From England."

The mains arrived. Carolyn's ruby-red beef rump steak was half-smothered in bearnaise sauce, like a cream duvet over a burns patient. It was, she complained, both overcooked and drowned. My lamb shank was huge, the size of a club, and darkened by long slow roasting (either that, or it had been left too long in the warmer), but it was strangely tasteless, lacking that unctuous richness that's teased out of lamb only gradually. I didn't get the impression it had been roasting in its juices very long.

The parsnip purée on which it rested seemed fine, but the spinach was a disaster, both soggy and tart with a nasty vinegary aftertaste that nagged at my insides all night.

We shared a perry and blackberry jelly, one of those wobbly rings of transparent sweetness invented by Mark Hix, and got the bill in a frantic dumbshow. By 10.30pm, the place was packed, a DJ racket of house music was on a rising crescendo and it was getting impossible to talk. Outside, 50 smokers huddled on the step, nursing pints of lager. At 8pm, the Mansion had seemed a rather groovy lounge, serving posh beers and cocktails to the jeunesse dorée of the southern metropolis. Now it had become like any other raucous boozer, and all fancy notions about "dining out" had been abandoned.

It's hard to make that transition from the pub with the menu chalked up on a blackboard, to the gastropub with printed menus and some château wines. The Mansion has both of these latter, but has neither enough privacy for serious diners, nor enough kitchen skill to attract a clientele beyond the chilled-out lager drinkers.

I'm sure they'll get there. For the moment, I suspect it's a place that does very nicely for not-too-demanding Sunday lunchers. I noticed they offer Sunday roast beef and Yorkshire pudding for £12.95, which includes cauliflower cheese, glazed carrots and spinach – but I'd steer clear of the spinach if I were you, unless you want to spend the evening swigging Gaviscon ...

The Mansion, 255 Gipsy Road, Dulwich, London SE27 (020-8761 9016)

Food 2 stars
Ambience 2 stars
Service 3 stars

About £60 for two, including wine

Tipping policy: "On tables of six and over there's a discretionary service charge of 12.5 per cent, all of which goes to the staff; all tips go to the restaurant staff"