The Habit of Art, Alan Bennett’s first new play for five years is a brilliant, witty and highly enjoyable meditation on the ethics and methods of biography, friendship in art, and the business of putting on a play. It’s deliciously funny, too, on the subject of the love that cannot speak its name because it has its mouth full.

The poet W H Auden (Richard Griffiths, all baggy and bitchy, like a monster prickly pear) is waiting in his chaotic Oxford rooms, designed by Bob Crowley, for a rent boy to swallow his pride before dinner. On a higher level, in every sense, the composer Benjamin Britten (Alex Jennings, uptight, prissy and over-sensitive) is auditioning choir boys for the role of Tadzio in his forthcoming, and last, discreet opera of gay yearning, Death in Venice.

It is 1972 and Bennett has ingeniously engineered a fictional reunion of the two friends after 20 years. Even more ingeniously, this is contained in a play-within-a-play in the rehearsal room where Frances de la Tour’s world-weary stage manager Kay is “running” the script in the absence of the director who is attending a conference about regional theatre in Leeds.

Auden is interrupted by the arrival of his future biographer (and Britten’s), Humphrey Carpenter – “I’m not a rent boy, I was at Keble” – played with glorious, scruffy intensity by Adrian Scarborough whose idea – as the “actor” playing Humphrey – of bolstering the show, and his own contribution, is to open the second act as Douglas Byng in drag singing “I’m Doris the goddess of wind”.

There are two great conversational scenes between Auden and Britten but Nicholas Hytner’s superbly light-fingered production is really an exercise in the open-ended, provisional nature of all theatre, with a rousing defense of the medium, and of the National itself, in Kay’s encomium to one of Bennett’s early directors, Ronald Eyre.

And exploiting Auden’s obsession with The Tempest, the rent boy Stuart (Stephen Wight) is allowed his moment in the sun, justifying the play’s alternative title of “Caliban’s Day”.

“Why does a play always have to be such a performance?” is the ambiguously inflected question represented on stage in the impatient, tetchy figure of Elliot Levey’s script-shuffling author and the hilarious speeches of talking furniture “filled in” (actors are missing at a Chekhov matinee next door) by Kay and John Heffernan’s effetely unselfconscious ASM.