The stage for Rachel Kavanaugh's production of this fine Alan Bennett play - a Leeds/Birmingham co-production twinned with Arthur Miller's A View from the Bridge, which will travel in the opposite direction up the motorway - is filled with a massive crimson quadrant of staircase rising seemingly towards the heavens. At the play's opening, George III's sumptuously panoplied court makes its entrance from the top, for all the world like a panto's walkdown finale. And sure enough, when the soundtrack cheers subside, the audience is found to be applauding before a word has been spoken.

It's a point superbly made, and Francis O'Connor's handsome design constantly underscores this contrast Bennett draws between the luridly coloured antics of the court - egged on by Stewart Wright's gloriously foppish and petulant Prince of Wales, desperate to shepherd in a regency of "style" - and the dour fustian of the politicians and medics who wrestle with the implications and consequences of the king's increasingly bizarre behaviour.

It might be thought that a reign which encompassed regular wars against France and Spain, the loss of the American colonies, the introduction of a national income tax for the first time anywhere in the world and the drawing up of the rules of cricket would be enough to shake the equilibrium of any monarch. As George III's bunch of quacks, Timothy Kightley, Tony Turner and Ian Barritt are reminiscent of the Marx Brothers, constantly peering into his urine, examining his faeces and taking his pulse, only to compound with useless nostrums the agonies of a man suspected by modern medicine to have been a victim of porphyria.

Willis, the sinister priest-turned-medic (a distinctly menacing Ken Drury), prefers horse-breaking techniques and is fortunate to coincide with a remission in the king's illness. Meanwhile, Pitt the Younger (Paul Raffield displaying political chicanery as to New Labour born) fights to maintain that the king is not mad but indisposed, aware that an incoming regency under the Prince of Wales would mean the death of his government.

Michael Pennington's George III is a towering performance. As a king with all the trimmings, he's imperious but benevolent, keenly aware of the failings that led him to lose the colonies yet in full control of his own domain. He struggles manfully to understand and to master his illness, which starts with a stammer, eventually developing into a gibber and total mental and physical incontinence. Yet, even as he succumbs, Pennington is simultaneously entirely credible as a man with his intellect intact despite the delinquency of his body. It is an enormously delicate portrayal and very moving indeed.

Strict historical accuracy is not one of Bennett's major concerns, which means he can trick out this well-constructed piece with plenty of characteristic quips and wordplay. He does justice to his subject, and this strongly cast production does him full credit.

- Ian Watson