It is rare for me to write a love letter to a London restaurant, but Joe Allen, which is 40 this year, deserves it; if you have any sense you will throw off misery and go there now for hamburgers. It is not really a London restaurant, which may be why I love it, but a Manhattan restaurant (established on 46th Street in 1965 by a man called Joe Allen) that was transplanted to London in 1977; the idea of Manhattan, anyway, which is more vivid in imagination than in life. I like to imagine the cast of All About Eve in Joe Allen, talking nonsense about ‘the theatre’ as they did in Joseph L. Mankiewicz’s words, because they were possessed. (‘You’re too short for that gesture.’ ‘What a dull cliché.’ ‘I hate men.’) It is certainly the best ‘American-style’ restaurant in London — although the Colony Grill Room is wonderful — and it is owed a kind review.

The entrance is pleasing: a fierce red awning on Exeter Street in Covent Garden, just around the corner from the Lyceum Theatre and its eternal tale of absolute monarchy with singing lions. This part of Covent Garden feels like backstage, which is thrilling by itself; the action — the lion — is singing somewhere else. There is a heavy door and a staircase to the cloakroom; you do not visit Joe Allen, you tumble into happiness. Very few restaurants can sustain this sense of excitement over many visits: the Delaunay is one, the Diamond Jubilee Tea Salon at Fortnum & Mason another; the Foyer and Reading Room at Claridge’s is a third, although I am not sure there is much reading done, unless you think the FT counts.

The room is a cellar, but charming: wooden floors and high ceilings; brick walls with mirrors; fragile paper tablecloths; playbills; photographs; the famous upright piano, played for many years by Jimmy Hardwick, who always smiled as you walked past.

Joe Allen pre-dates London’s restaurant boom, and the city is full of its imitators and alumni; that is why it feels familiar. Rowley Leigh was the chef; Jeremy King — who opened the Colony Grill Room — was the -maitre -d’; Graham Norton was once a waiter. (Whether he was a good waiter, I cannot imagine. It could have gone either way depending on his mood.) That is why you might think Joe Allen is nothing special, but sit and eat and you become aware that you are in a masterful restaurant that knows what it is doing. Musical theatre stars don’t come in in groups each night any more — I only saw tourists and day trippers, grateful the monster city wasn’t robbing them of their gold — but that doesn’t matter. They are on the walls, and perhaps it is safer that way.

I have a test for Joe Allen, surely; it is a Saturday lunchtime and I have brought three children under nine. They are quite soon under the table and screaming for bread — but that doesn’t disturb Joe Allen, who has looked after actors for years.

The food, of course, is American classic, which is simple but hard to do very well: grills, eggs benedict, fish from the market, a famous caesar salad. It comes swiftly. The three-year-old is coaxed from under the table, the six-year-old dreams of Rada, whose graduates stare down by the loo, charismatically, and everything we eat — hamburgers (they are not on the menu, but we asked for them and they came), a half chicken with lemon and rosemary, a quartet of thick, dark chocolate sundaes with raspberry — is fine, and all for £90.

As we leave, the nine-year-old tells the pianist he is learning to play the piano; suddenly they are playing a duet. In a city that feels to me ever more malevolent and covetous, these things matter. It feels strangely like grace, and so I wish Joe Allen a happy birthday.