The Madness of George III provides a devotional, slightly sentimental view of royal pluck and resilience of the sort that you would expect to find either at Chichester Festival Theatre or in the heart of the West End.

The play, with a cast of 25 actors, is conceived upon the kind of epic scale normally reserved in these pound-pinching times for musicals with millions to spend. The very first scene proclaims the style. Down the great flight of steps which occupy the entire width of the Lyttelton stage, as designed by Mark Thompson, comes a royal procession, moving to the music of Handel. A botched assassination attempt upon George Ill's life is brushed aside by the genial, extrovert monarch who airily observes that the would-be assassin is evidently out of her mind.

This incident achieves a reverberating irony in the play; almost at once royal ceremonial gives way to disorder: George, in Nigel Hawthorne's racking performance, begins to go spectacularly out of his mind � and how. As Bennett shows him suffering a nasty repertoire of symptoms, from seizures and blisters to mad talk and purple urine, the pomped-up doctors show none of that compassion the king bestowed upon his aspirant murderer. The play is not, however, a condemnation of doctors, by turns cruel and stupid, as they brood over bleeding and heating ploys to cure a king whose power falls swiftly away. Bennett does poke late 20th century fun at the absurdities of these men � bewildered by what we now know to be the hereditary disease, porphyria: Harold Innocent's suave blusterer, Cyril Shaps' Sir Lucas Pepys, whose interest is morbidly confined to the bowels, are delightfully fatuous examples of doctors gravely reduced to chronic quackery.

But Bennett is as interested in the dynastic and political intriguing which the king's great illness engendered, the power-games which the prim Prime Minister Pitt (Julian Wadham as a human ice-mountain) plays against the King's heir, the Prince of Wales, in alliance with Charles James Fox. And Nicholas Hytner's invigorating production is staged as a series of grand set pieces in which court ritual is remorselessly overwhelmed by madness's disorder, Janet Dale's ineffectual German queen exiled to the sidelines. And the closing of the first acthorrifyingly images this method; to the exultant singing of Handel's Zadok The Priest, the dishevelled, trampish king, looking like a refugee from Samuel Beckett country, is tied to restraining chair and his screams are overwhelmed by the music.

Bennett's sympathies are surprisingly royalist-traditional, devoted to the king and his Prime Minister rather than the heir and Fox. And his feelings causes him to play pretty loose with what historians like Plumb and John Brooke tell us: Michael Fitzgerald's Prince of Wales is ridiculously got up as a vacuous, callous intriguer; the great Charles Fox is here not much more than a hearty opportunist rather than the man who attempted to reconcile monarch and the son.

In the last resort the play celebrates � a touch too lengthily � a royal spirit refusing to go under. Tortured by experimenting doctors, incoherent, this George emerges at last with his sanity restored � a Lear who comes through. And Nigel Hawthorne's monarch, in his right mind or out of it, has a flinty, fiery pathos. We were, in the best sense, quite amused.