“It is difficult to write a scene in which a young man has his first deep experience of sex with a girl, is fully satisfied by the event, then gets up and blinds a lot of horses.” Thus did playwright Peter Shaffer describe the tricky psychology of Equus, his shocking and wildly successful 1978 play now being revived in London.

The answer, he realised, was that the boy was prevented from consummating sex with the girl by a prior and passionate attachment to the horses in his care — a condition for which he receives intensive psychotherapy. Equus was a sensation on both sides of the Atlantic — for its nudity, for its cruelty to horses (English audiences) and for its alleged misrepresentation of psychiatrists (New Yorkers).

No stranger to controversy, Sir Peter has also written Amadeus, in which a farting, giggling, capering young Mozart drives his mentor, Salieri, to despair, and The Royal Hunt of the Sun, a savage indictment of Spanish imperialism. Both these and Equus were made into films, with Amadeus winning eight Academy awards.

He could be funny too, notably in Black Comedy, a farce in which a blown fuse leaves characters feeling their way around in the dark, prey to mistaken identity, social gaffes and sexual possibility. The actor Simon Callow said of Shaffer: “His great skill is the distillation of character into one theatrical idea.”

Peter Shaffer was born, with his twin brother Anthony, 80 years ago to a Jewish family in Liverpool. Both were Bevin Boys in the war — sent down the mines as an alternative to National Service — and both educated at St Paul’s School and Cambridge. They even wrote a novel together but Peter was the first to become a playwright; later, when Anthony left a career as a barrister to write plays himself, Peter felt threatened and begged his brother not to use the same surname.

Until the age of 32, he said, he had felt “one of the boys, never quite unique”. Now he was in danger again of being “part of that faintly cute and annihilating ‘Which one of them did it?’ ” In the end, the two managed separate, distinguished careers — Anthony, three times married, in England and Australia; Peter, never married, in New York, where he has lived for many years, though always premiering his plays in London. Anthony’s death from a heart attack in 2001 “depressed and distressed” his surviving twin.

Despite several revivals of his plays during the Nineties, Sir Peter has not produced anything fresh since 1996. He has had plays in “manila folders, half written” for some time but confessed: “I feel slightly like that donkey in Aesop’s Fables that can’t decide which pile of hay to eat, so eats neither and starves to death.”