Le Chabanais is a long slim room lined entirely in foxed, patinated brass, like a safety deposit box forged from old gold / Daniel Hambury/Stella Pictures

8 Mount Street, Mayfair, London W1 (020 7491 7078)

A new restaurant has opened across the road from us. Seasons changed as the site, formerly a moribund chain restaurant, was slowly refurbished. Builders and shop-fitters came and went. We were literally watching paint dry. There were problems with the extractor fan and waste collection service. When we finally bowled in to Megan's for a first meal, we were already familiar with the team, having watched them open up at 7am and wearily turn off the lights at 11pm.

This corrective reminder of how tough it is to open any new restaurant should be on the core curriculum of all critics. We sail in during opening week and pick places apart, ignoring the fact that it's a miracle anywhere ever opens at all.

Le Chabanais, an eagerly awaited restaurant in Mayfair, was one of the rare openings that go wrong. A London offshoot of the influential Parisian bistro Le Chateaubriand, it opened for a week back in May, then immediately closed, due to "mechanical difficulties" – one of those euphemisms, like "creative differences", which hide a mountain of pain. All bookings were cancelled – and the place was fully booked, as curious foodists flocked to see how Inaki Aizpitarte, one of gastronomy's shooting stars, would adapt his philosophy of casual fine dining, forged in branché Belleville, for a Mayfair audience.

The prince of bistronomie he may be, but there's nothing bistro-ish about Le Chabanais, plushly situated on Mount Street, between the Connaught and Berkeley Square. It's a long slim room lined entirely in foxed, patinated brass, like a safety deposit box forged from old gold. A central bank of round tables offers opportunities for discreet business conversations, but the wall-side seating is more cramped, and it's hard to imagine the tall marble counter catching on with the suited-and-looted locals.

It took me two lunches to get to grips with the place, and even now, I'm not quite sure what to make of it. Head chef Paul Boudier, transplanted from Le Chateaubriand, spins some fantastic, vivid dishes from mainly British produce. Plates come swaddled in leaves and flowers, and herbs are used with particular delicacy, as with the citric rush of ground ivy which punctuates a dish of crisp-skinned guinea fowl and crushed purple potatoes. Silky ravioli pouches filled with rough-hewn chicken liver are bathed in a limpid fennel broth humming with anise. The sweetest raw langoustine comes paired with sea vegetables and a pungently chlorophyllic lovage sauce. Sweetbreads are tossed with nubbles of sesame seeds and a succulent swoon of miso-glazed smoked aubergine.

For every outstanding dish, though, there's a miss, or near-miss. Turbot, as sexily dressed as anything from the neighbouring Roland Mouret shop in ribbons of shaved courgette, surrenders to an overpowering foamed butter sauce. A vivid pea soup comes pointlessly sprinkled with coffee granules. An undercooked poached egg oozes albumen over sautéed girolles and sorrel leaves. Lobster is paired with cannellini beans, spinach and redcurrants in an imperfectly clarified broth: all a bit squeaky and ascetic.

Weirdest of all are the desserts, which read normal, but taste like the pastry section has gone on a spree through the veg chef's mise-en-place. Buttermilk ice cream comes with sage; cherry sorbet is jarringly paired with dehydrated black olive, and the signature Mont Blanc adds sliced raw mushroom and some kind of dried mushroom powder to the traditional chestnut purée and whipped cream, a challenge on a par with scaling the actual Mont Blanc.

And they don't really do challenging round these parts. The Mayfair crowd, at least at lunchtime, is dominated by suity gents. They're talking fracking and frocks; they're not interested in foraging. This is ingredient-led, natural food in the least natural postcode: Hackney for hedge-funders.

The identity crisis is compounded by the bar downstairs, which on both lunchtime visits was filled with people tapping away on laptops, like an upmarket internet café. Perhaps they were associates of the backer – a London-based Indian film producer– tracking their investments.

Still, it's a beautiful room (keep it positive, remember). And the staff are relaxed, clued-up and enthusiastic – one of them so enthusiastic about the selection of natural wines that he recommended a white costing £21 a glass.

It may well be that devotees of Scott's and 34 will relish the chance to try a more edgy style of cooking. But I fear the clue is in the name. The Parisian mothership, Le Chateaubriand, is named after a prised cut of beef designed for sharing, promising conviviality and satisfaction. Le Chabanais references a famous Parisian luxury brothel: an expensive way to part rich men from their money.

Food ***
Ambience ***
Service ****

Le Chabanais, 8 Mount Street, Mayfair, London W1 (020 7491 7078). Around £55 a head before wine and service