The B&H Garden Room, Assembly Hotel, 31 Charing Cross Road, London WC2H 0LS (020 3962 7275). Starters £9-£12, mains £14-£32.50, desserts £8-£12, wines from £27

The moment I’d paid the bill at the Bourne & Hollingsworth Garden Room atop the Assembly Hotel, I took the lift down 10 floors and lolloped up London’s Charing Cross Road in a blind, pigeon-toed fury. I went to the Ivy Club, the shiny members’ space above the theatreland restaurant of which it is a part. Yes, I’m a member. So shoot me. They mix a killer whiskey sour, there’s an impressive live jazz scene and they have a no-photography rule that means I can get drunk and slip off a bar stool like treacle spilling from its tin, and no one will have pictorial evidence.

This evening I wasn’t there for the jazz or the inebriation. Even though I’d already had three courses, I wanted an Ivy dessert; one so famous that, despite, being off menu, it’s still a big seller. The kitchen at the Garden Room had taken the classy idea of that dessert and committed the culinary equivalent of grievous bodily harm upon it. They had kneed it in the metaphorical knackers and laughed in its face before serving it to me like it was an entirely reasonable thing to do. I wanted to be reminded just how good the real thing is, and how bad that one had been. I wanted to nurse my anger.

I just wish it had been the only sin committed against good taste and good manners that evening. It wasn’t. As ever I had expected so much more. This is because I’m a fool. Bourne & Hollingsworth is a self-styled “creative lifestyle company”. They “curate beautiful spaces”. They run brasseries and bars and pop-ups and “happenings”. They admit they “aren’t for everyone,” which is certainly true. Based on this meal, they are not for me.

A few months back they took over the top floor of the Assembly, which is one of those bare-bones hotels where they make you use your room card to access the lifts so you feel secure. Only the 10th floor is available to the public. Normally, I stick to a mantra: never eat in a restaurant that is on a boat, that turns or has a view because it won’t ever be about the food. But B&H talks a good game, and I adore London’s roof tops, those unexpected views of hidden terraces; of windows turned to the sky; of skyscrapers piercing the clouds and, in the distance, the place where the city gives way to the country. The Garden Room view really is spectacular and on a summer’s evening, as the sun dips behind the capital’s glass cliff faces, it feels like a special place to be.

Then the nice waiters start bringing the terrible food. It is a brutal exercise in portion control and failed staff motivation. I genuinely don’t think the kitchen wants to be this clumsy and graceless; I just suspect they’ve been given no particular reason to do otherwise. We start with king prawn and chestnut mushroom skewers with béarnaise sauce, which bring to mind the old Jewish joke: “Oi, the food is awful, and the portions, so small.” For £10.50 you get four tough and underseasoned prawns and three flabby and undercooked mushrooms. Weirdly, the béarnaise seems to have wandered in from another kitchen; one where someone cares. It’s thick and frothy and rich with fresh tarragon.

There is only one way to deal with this trauma. Order the crispy kale salad. The new trauma will deaden the old. The crispy bits appear to have been deep-fried a long time before we made the mistake of coming. They are cold and leak vegetable oil into your mouth as you bite in. There are also soggy leaves of undressed, steamed kale; thuds of green matter, waiting to be composted. Occasionally you uncover a piece of roasted carrot that is mildly pleasant. There are nuts which, like my hopes and dreams, are crushed. It costs £10.

The menu is the equivalent of eager dad dancing. It’s attempting to find the cultural pulse without ever understanding the things it’s trying to do. Just putting pomegranate seeds on to a roasted cauliflower salad does not make you Yotam Ottolenghi, especially if the vegetable is barely coloured. It looks like once-boiled cauliflower that’s been resting in the back of a fridge, perhaps in a recycled ice-cream tub, looking for a purpose. It’s the sort of dish you knock up for your offspring’s new vegan partner because you don’t like them very much. It costs £14. Add a meagre portion of cotton wool chicken and you’ll be paying £19 for the word dismal. The best dish of the night is a leg of duck confit, with an overcooked celeriac and potato gratin and a tooth-achingly sweet cherry sauce. It’s food fit for a banquet. By which I mean the banqueting suite of a regional ring-road hotel where they’re handing out awards for innovation in quantity surveying.

And so to dessert, because this restaurant reviewing lark is a job not a jolly and I have to eat things I don’t like. There is a chocolate marquise, which has completely split. They’ve attempted to disguise this by smoothing over the disc of set chocolate with a hot palette knife, but spoon inside and it’s grainy, like furiously overwhipped cream.

The other dessert, the one which sent me running up the Charing Cross Road, is listed as iced berries with white chocolate sauce. The version served to me at Ivy HQ, where it first became famous decades ago, is exactly as it should be: a large plate spread tightly with frozen fruit. A modest amount of hot, white chocolate sauce is then poured on at the table. It’s sweet and sour and crunch and burst and hot and cold and clever.

Here, it’s a deep bowl of cloying white chocolate soup, bobbing sparsely with abandoned berries which have lost their chill and are drowning. It’s a dish seemingly prepared by someone who has never met the original. It makes me cross. As, by now, does the scuffed white faux-wooden floor, and the grubby beige linen arms to the chairs, and the treatment of the other diners, who will probably spend £60 a head here and deserve so much more than just the view. I pay. I punch the lift buttons and head to my fancy members’ club. Most of the time it’s an extravagance. Most of the time it’s where I kick back. Tonight, with a spoonful of perfect iced berries and white chocolate sauce in my hand, it’s a place of safety.

News bites

Not far away from the Assembly on Leicester Square is another budget hotel with a rooftop bar which is worth the effort, because they’ve kept the offering simple. The LSQ Rooftop, at the Indigo Hotel on the north side of the square, has a pleasing menu of small plates, mostly priced around £8: sautéed mini chorizo sausages, fried buttermilk chicken, king prawns with chilli and so on. All that, solid cocktails and a great view too (lsqrooftop.com).

Former Professional Masterchef runner-up Matt Healy, whose Leeds restaurant I very much enjoyed recently, has taken over a pub just outside the city. The menu at the Beehive in Thorner. includes their own pork pies, corned beef and haslet, alongside more restless dishes including linguine with lobster broth, mussels and clams, and tiger prawns with Asian salad and dipping sauces (mhbeehive.co.uk).

Dishoom, the Bombay-inspired small restaurant group, with five outlets in London plus one each in Manchester and Edinburgh, has a new baby: its own cookbook. Dishoom: from Bombay with Love is by founders Shamil and Kavi Thakrar and their executive chef Naved Nasir and is published on 5 September. They promise it includes the recipe for their breakfast bacon naan (dishoom.com).

Jay Rayner will be appearing in a special Guardian Live event at London’s Cadogan Hall on 9 September. In My Last Supper, the show accompanying his soon-to-be-published book of the same name, Jay examines our fascination with last meals and tells the story of his own. Click here for tickets.

Email Jay at jay.rayner@observer.co.uk or follow him on Twitter @jayrayner1