The Habit of Art, Lyttelton, National Theatre, London

For many years, Alan Bennett was accustomed to stand on the threshold of the closet, pointing a Hush Puppy first one way, then another. But with the advent of love in the shape of Rupert (World of Interiors) Thomas, the partner who offered him vital support during his fight with cancer in the mid-1990s, Bennett has been prepared latterly to nail his colours to the fence.

The man who once said on television that “be yourself” is a “baffling injunction” is in no danger of belting out “I Am What I Am” even in the privacy of the shower. But, on the drama front, he has not only abandoned the closet, he’s proceeded to chop it down into metaphorical matchsticks. These he has redemptively transformed into the Eighties school with its clever Oxbridge class and their maverick teacher in The History Boys, the mega-hit that earned the National Theatre a cool £ 5 million, and now, in his hotly anticipated and hilariously provocative new play, The Habit of Art, as a location that looks suspiciously like the National’s Rehearsal Room One.

We meet a group of actors who, abandoned for the day by their director, are staging a run-through of a new play. This inset drama focuses on the poet W.H. Auden just after he’s returned from half a lifetime’s exile in New York for what turns out to be a lonely, unpopular retirement in his old Oxford College, Christ Church. He is visited by a young journalist, Humphrey Carpenter (Adrian Scarborough), the real-life writer who would go on to be his biographer. In Bennett’s farce Habeas Corpus, there’s a sequence in which the wrong women is professionally touched up by a false-breast-fitter, with the result that her hibernating libido awakened. In The Habit of Art, the high-minded Carpenter is mistaken for the rent boy whom Auden is expecting with a view to fellating before supper at High Table in hall. But it all proves to be about as erotic as the mixing of one of the poet’s draconian dry martinis. Ruled by the clock, the older Auden on the whole preferred punctuality to penises and so the rent boy (Stephen Wight) is cheated of the oral skills of the author of “The Platonic Blow” by the chimes of Great Tom.

Because of the play-within-a-play structure, Richard Griffiths, heroically and superlatively replacing the indisposed Michael Gambon, plays both Auden and Fitz, the mountainous, heterosexual, amiably forgetful actor who has taken on the role. The incongruity is wonderfully entertaining, rather as though nice, equable old Arthur Negus had been cast as Genet. Overseen by the young tetchy, trainers-wearing author (an adroitly narked Elliott Levey), the inset drama is set in 1972 and stages an imaginary meeting, after a gap of twenty-five years, between Auden and his erstwhile collaborator, friend and psychological protégé, the composer Benjamin Britten. The latter is excellently portrayed in both his comic bassoon-up-the-bum inhibitedness and his tragically recessed self-repression by Alex Jennings who also plays his portrayer Henry, the kind of gay man that “trade” might call “a bit of neat”.

The title refers to the dedication to their craft of the two elderly maestros and the encounter between the sick but still productive Britten and the on-half-throttle but emotionally more honest Auden is ignited by the news that Britten is at work on Death in Venice, an opera based on Thomas Mann’s novella about an ageing writer’s obsession with the youth Tadzio. There’s an implicit contrast between this Britten opera - which with its arty symbolism and Apollonian evasiveness feels like a case not so much of self-outing as of putting a cat-flap in the closet door after the moggy has bolted - and the rollicking irreverence and recklessness of The Habit of Art, which mixes hard-won wisdom about such matters as the meaning of collaboration, the dubious value of biography (Carpenter wrote a life of Britten, too) and flurries of delirious silliness.

Nicholas Hytner directs with an unerring instinct for the volatile nature of the material in a cracking production that flirtatiously keeps the audience up to speed with the outrageous amount of information and allusion. The play-within-the-play is replete with potty, poetry-spouting personifications of, say, Auden’s fabled Wrinkles who kvetch about the strain of working on a face that, as Hockney once remarked, made you wonder how on earth his scrotum looked. The pastiche is always superbly pointed. I loved the moment when Britten, barking avuncular orders at one of his beloved boy sopranos, pounds strenuous pianistic dissonance into one of his nursery rhyme settings. The outer play is full of lovely observant comedy about the protocols of the rehearsal room, especially from Frances de la Tour all dryly witty, battle-hardened, managing motherliness with the egos who have landed in Rehearsal Room One. Hytner revealed at a press conference that Bennett at one stage wanted to call the play Caliban’s Day and you can see why. For just as The Sea and the Mirror, Auden’s poetic meditation on The Tempest gives the last word to the low-class monster, so Bennett allows the rent-boy to speak up at the conclusion for the culturally excluded bit-players who service the educated but don’t get a look-in at life’s ongoing arts festival. “I don’t even know what I don’t know,” he complains. “I want to get in. I want to join. I want to know”.

At certain points appearing in a rubber Auden fright-mask, Griffiths enhances both layers of the show by coming wreathed in his own micro-climate of tolerant humanity. But throughout the first half I was bugged by the fact that Bennett’s characterisation reminded me of someone outside this world. Then it hit me: he’s infused his version of the poet with some of the key qualities of his quondam best friend, the chat show host Russell Harty. Bennett’s address at Harty’s memorial service is a masterpiece both of affectionate recall and of stealthily mounting indignation at the gutter press’s gay-hating intrusion into his final illness. Lacking Bennett’s fine-spun scruples, Harty, a fellow-undergraduate, had taught him several early lessons that the dramatist would spend years improving from theory to fact “He was uncensorious of himself and of other people. He knew that there are no rules”. He knew too that we are born into knowingness. And so it is Harty I hear when Auden acutely abolishes the victim/victimiser distinction in his interpretation of Death in Venice and adds a new note to the chord that masses in the play on the significance of one of its central themes. Of Aschenbach and Tadzio, he says to Britten “Why does innocence come into it? Neither of them is innocent. It’s not corruption. It’s collaboration. How fondly, one fancies, Harty would have enjoyed seeing his old friend “fixed up” with love and to have witnessed this play and the discreet depth of its tribute to him. Thanks to his benign influence on this new Bennett play, we are now all in his debt - and so Russell Harty, from my side of the Styx to yours, I would like to shake your hand.