Lecture Room and Library at Sketch9, Conduit Street, London W1

Oh blimey, it's that time of year again; my annual dinner with the highest bidder in The Independent's charity auction. I always find these blind dates a bit stressful. One of our generous readers has paid money to come out on a review with me. The restaurant had better be bloody good. And I had better behave myself.

This year's winner, Philip Turton, placed his bid as a surprise birthday gift for his wife, Alison, and was keen to find a dinner destination with the suitable wow factor. We finally alighted on the Lecture Room and Library at Sketch. From a price point alone, it fitted the bill, though it isn't so much the wow factor as the wtf? factor – with starters from £35 to £45, and main courses from £37 to £55, it is surely Britain's most expensive restaurant. But this was a special occasion, right? And Sketch is celebrating its 10th anniversary this year, so there was an editorial justification, too. (No, of course I didn't just suggest it as a way of getting to eat at Britain's most expensive restaurant...)

In the decade since I reviewed The Gallery, the less formal of Sketch's two restaurants, I've only visited this mad Mayfair pleasure-dome a few times. And I'd forgotten just how amazing it is, in every sense. A Georgian townhouse captured by situationists, it feels more like an art happening than a restaurant, with fake blood spilling up the stairs, disconcerting sculptures peeking from alcoves, and on a Saturday night, a dazzling cast of international totty, wobbling around in teeny dresses. The Gallery, which I remember as an all-white space, is now a frenetic mash-up of clashing stripes and ziggurats, after a recent redesign by artist Martin Creed.

The Turtons were huddled in a corner of The Glade Bar when I arrived, dodging the tiny red lights which strafed the walls like the laser sights of a sniper's rifle. "It's not quite like anywhere I've ever been before," gulped Philip, a barrister who specialises in personal injury cases. Given the lowness of the lighting, the strength of the cocktails and the height of the heels, there might have been some professional self-interest in his choice of venue.

After some confusion about our booking, which led to us being briefly seated in The Gallery, and I saw my whole life flash before my eyes, we were eventually shown up, beyond the velvet rope, to the Lecture Room. Demure and calm, in contrast to the bedlam below, it's a beautiful and distinctly grand space.

Given the noise and flash of the Sketch experience, the food really needs to be spectacular to make any kind of impact. And it is. Under the direction of the great modernist Pierre Gagnaire, the menu reads like a collaboration between a student poet and a random ingredient generator. Each course is a tasting menu in itself, comprising three or four dishes, which riff on a theme then veer off into something extraordinary.

Take Philip's starter, which is called, in all seriousness, Perfume of the Earth, and is apparently inspired by Gagnaire's favourite scent, Terre D'Hermès. First a casserole dish , smoking like a witch's cauldron, is brought to the table, and the lid reverently lifted to release a tobacco-ish, almost incense-like scent.

Its contents, a complicated stew of snails and vegetables, are returned to the table to be anointed with a foie gras soup. A bright green lozenge reveals itself to be a parsley ice cube. A flag-like arrangement of jellified vegetable squares conceals a black olive jelly and a creamy nugget of burrata. And to complete the dish – why not? – an almond and root vegetable tart, so sweet it seems to have wandered in from the dessert trolley. Philip is incredulous, delighted and bemused in equal measure.

The 'one weird thing' rule applied to other dishes; the satire-defying peeled grapes stuffed with olives and anchovies that came with Alison's saddle of lamb, along with a little tartlet of lamb's sweetbreads. Or the rhubarb tart topped with sliced mushrooms which mysteriously accompanied my raspberry soufflé. ("It's mushrooms," shrugged our waiter, as though that was the most natural thing in the world.) Sometimes these oddities worked brilliantly. But sometimes they just seemed a bit random.

That said, Gagnaire's open-minded approach, particularly to Asian ingredients, creates some intriguing high points. A cocotte of Iberico pork chop comes with a salad of pig's ear and kimchi. Dover sole is dusted with toasted hazelnuts and bathed in a velouté of black rice. And there is some serious technique at work behind the riffing on temperature and texture, courtesy of head chef Jean Denis Le Bras. A scallop starter offered slices of pan-fried scallop with a liquorice/morel cream; a separate dice of morels and raw scallops marinated in Arbois wine; an 'ossobucco' of sweetbreads, and a jolting sorbet of grapefruit and chilli. A chocolate-themed dessert is like the practical in a maître-pâtissier's final exams, comprising a soufflé, a pistachio parfait, a ganache and a tuile (all superb).

Surrounded by the multiple dishes that made up each course, we found ourselves uncertain whether to mix and match, or doggedly work our way around, as in a gastronomic game of clock patience. We didn't get much clue from our waiter, who affected the airy disinterest of the professional announcer, rather than really engaging with our experience. Dishes were introduced in a heavily-accented murmur – and there's only so many times you can make a chap repeat "wasabi financier" before one of you cracks.

Still, the Lecture Room does feel more playful and relaxed than most Michelin-starred joints. At one point, Alison emerged from the loo wide-eyed, having shared the space with what she described as a "porn star". You don't get that at Le Gavroche. In a weird way, the through-the-looking-glass strangeness of it all helped to heighten the sense of adventure and discovery. We didn't know each other, we didn't really know what we were eating, but it all added up to a rather fantastic experience. The kind of meal that happens once in a lifetime, rather than just once a year.

Lecture Room and Library at Sketch9 Conduit Street, London W1 (020-7659 4500)

Around £110 for three courses a la carte, before wine and service

Food *****

Ambience ****

Service ***

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"