The Piazza is not a piazza – a realisation which is always irritating – but a restaurant in the eaves of the Royal Opera House, now restyled and open to those without tickets to the opera or ballet. If it were honest, Piazza would name itself Attic or Eaves, but the Garden, as idiotscall it, has long been a slave to delusions of the most boring kind. (It is no longer a garden in the wreckage of Inigo Jones’s square. I wish it were.) I would be happy to dine in a restaurant called Eaves – my favourite hotel is a hole in a wall by the Jaffa Gate in Jerusalem and my favourite restaurant was a man with a fish in Jamaica – but, in London, even attics are not what they ought to be.

I once spent a week backstage at the Mariinsky Theatre in St Petersburg, watching ballet dancers not eat, and in the eaves I found a piano graveyard, a tutu graveyard and, further on, a musical-score graveyard. Those are eaves to be proud of, but the people there are still obsessed with the siege of Leningrad, when a German shell hit the cupola and snow fell in the theatre: at least that is what they told me. I want to believe it, but I love Pyotr Tchaikovskyfor the violence of his flounces.

Here, at the Garden that is not a garden, they have no such dedication to their myth: rather they seem to long for John Lewis. Perhaps that is a peculiarly English thing, but it is not the Englishness I admire. I do not understand why the people who run the opera house strip out everything magical they can find.

Piazza is a new and carpeted nadir. At the ground-floor entrance, you are stared at and searched and then admitted: the days of idly wandering into London’s public buildings are gone. You need a QR code to approach a Rembrandt these days. You then climb the stairs – I daren’t peek into the auditorium, the atmosphere does not invite it – and then mount what feels like it is the world’s longest escalator: the sort you find in Dubai, or a Terry Gilliam dream. At the top there is a sea of neutral carpet, and a bog with 200 very clean seats. If the Phantom is here, he has been blinded by Pledge and bourgeois values, and has likely locked himself in a cupboard to die.

In a small, neutrally coloured restaurant, you find the kind of people who visit Covent Garden in the daytime: that is, retired people who think nearness to opera is posh. We ask to sit on the terrace, which is better, but it is glassed in: you can see the filmy towers of Holborn.

The food, of course, is posh, which means over-styled: the chef is from Per Se in New York City, which terrorises diners with minute and neurotic once-food. We eat tiny Dorset mackerel and a lump of purple smoked salmon. It seems likely that heritage beetroot has got to it – gang warfare on a plate – and it isn’t happy. It looks like a fishy bruise. I don’t see the point of serving heritage anything while trying to summon John Lewis, but this restaurant isn’t for me. It’s for people to want to spend £200 on two courses and stare at new carpets and then go home to stare at their own.

Beef on the bone is better, and I have to admit the truth: I never ate a tripled cooked chip I liked until now. But battered by the carpeting and the world’s least inspiring opera house attic, we are too sullen for pudding. The Garden of all places should know better. Where is the drama?