This is a dullish, crudish play to come from that elegant instrument Alan Bennet's pen; but at its centre is an unforgettable image. That is Nigel Hawthorne's George, the Lear of 18th-century Windsor, stripped to a shabby vest, strapped into what might be a prehistoric electric chair, wailing out commands as boiling wax is conscientiously poured on his head. Some of those around me suddenly developed and urgent need to inspect their programmes, their knees or their feet: anything but the human thing up there on the stage.

The play starts, as it should, with Hawthorne's George at his happiest. Up go the lights on Nicholas Hytner's production, the shiny floorboards of Mark Thompson's set raked back to a broad sweep of steps; and down comes the king, all energy and health. Before long, he has launched into the cheerily blimpish repetitions characteristic of him: "I've written pamphlets on agriculture, what, what, hey, hey?". He is that benign busybody, Farmer George. He cannot see a girl with a warming pan without telling her the coal inside comes from Kent.

Bennett is justified in admiring him. He was a far more active and able king than Whig historians sometimes suggest. Yet doubts are already surfacing. Bennett's work as a whole, Forty Years On to Single Spies and even Wind in the Willows, is a series of wry ruminations on the arcane oddities of England. But his plays have nevertheless gone in for gentle portraiture rather than crowded oils. Faced with introducing a divided royal family, wrangling politicians, corrupt courtiers and meddling physicians, his usual subtlety often deserts him.

There are too many lines obviously filling in the background. Just in case we fail to hear that homely Charlotte has 15 children, or notice she affectionately calls him Mr King, George must say things like "a man should marry, the Queen's a treasure, not a beauty, what what". But that is a smaller matter beside Bennett's handling of Whigs and Tories.

Julian Wadham's Pitt is an exorbitantly wintry workaholic, whose response to learning it is Christmas Day is "Is it? It's all one to me". But he is reality itself beside his enemies, a gloating, snickering retinue headed by David Henry's pontificating Fox ("there's a fountain of goodness in the nation, and polluting it is George Rex"_ and Michael Fitzgerald's Prince of Wales. That Georgie junior was fat and spoiled we know. But where is the evidence that he wanted his doctor to accelerate his father's death, or sneered at his agonies? Caricatures disingenuously cackle, "no danger, I hope". Frustrated heirs are more wary.

So with the physicians. We know they were babes in the wood, blundering into trees, and mistaking weasels for wolves and porphyria for "flying gout" or "simple dementia". We know they made George worse. But here they keep boringly rehearsing pet obsessions, whether blistering the skin or inspecting the royal stools. Only Charles Kay's Willis, a clergyman transformed into a Mengele-like behaviourist, holds the interest. He turns the court into a Skinner rat-maze, dizzying the king with punishments and rewards. The original was less blatantly sinister and religiose; but Kay's aversion therapy is horribly fascinating.

That is because of Hawthorne's acting, the evening's major success. He degenerates, knows he is doing so, and cannot stem the lewd chatter and weird fantasies pouring from his mouth. Often, he is a sane man observing his own eccentricities from a straitjacket. There is a moving pluck in the defiance with which he greets his ill-treatment; and, as with R. D. Laing's schizophrenics, his madness can make sense. Why should he not shake, stutter and burble at the mention of America? He is a responsible king, and thinks its loss his fault.

Bennett avoids accusations of sentimentality by brining on a modern doctor to remind us that the "cured" king of 1789 succumbed again to porphyria. But his usual wit seldom comes. The funniest exchange - George responding to Willis's justification of his change of trade, "our Saviour cured the sick", with "but not for �700 a year" - actually happened. Worse, Bennett's tale of a king used and abused by the Establishment hasn't the depth and resonance he must have hoped. He ends pointedly, with an extract from Shakespeare's greatest tragedy; but his Lear is no more than that sad historical figure, Farmer George.