THE TWO plays that he now has running on the South Bank stand as monuments to the furious frivolity of Alan Bennett. In the Olivier, the wind whistles once more through the willows, revealing a deep-set conservatism that culminates in the certainty that weasels can be routed and Toad shall have his hall again. In the Lyttelton, an altogether blacker vision unfolds of an establishment under threat from the enemy within, debarred by history from the possibility of a happy ending.

The Madness of George III is an exquisitely painful examination of a political disintegration that has quite unexpected parallels with our own times. George III, as every history student knows, talked to trees and lost America, not necessarily in that order. Bennett seems to have a- genuine affection for "Farmer George", who has not only his own madness to contend with but the impatience of the heir who would keep him mad and the ambition of the doctors who profit from that madness. As he sits, appallingly blistered and blabbering, denied his queen and his clothes, his government slides towards defeat by Whig intriguers who have adopted the dissolute Prince of Wales as their figurehead.

Where George had a civilised curiosity, "benevolent, undirected and infinite", the new order is cynical, dissolute and fixated with style. You can feel Bennett recoiling from it, much as he recoils from the oikish vulgarity of the creatures occupying Toad Hall, yet it would be simplistic to imply that his analysis runs along straight party lines. What emerges limpidly through this important new play is that his politics are governed by cast of mind rather than cast of vote: better a Tory toad than a Whig weasel.

King George might be good and true, but his Prime Minister Pitt (Julian Wadham) is a pallid nonentity, while his physician (Harold Innocent) is a profiteer who creates a run on the stock market by selling out on his own, confident prognosis on his king's health. For the opposition, Fox (David Henry) is a man of integrity unimpeachable but for the colour of his nose and the morals of the mistress on his arm. His disciples, the playwright Sheridan, and the statesman Burke, are at the very least men of substance.

At the heart of the play lies the connection between public and personal disintegration, which is magnificently underscored by Nicholas Hytner's operatic production. The pomp and ceremony of Handel combine with Mark Thompson's picture-frame setting to create a public dimension which is offset by the quirky intimacy of Nigel Hawthorne's king.

Whether playing happy families in bed with his knitting queen, or enduring the degradations of assorted quacks, Hawthorne remains unshakeably and touchingly human. Janet Dale provides him with a splendid consort: funny, faithful and frumpily Germanic, while Michael Fitzgerald creates a fat slug of a Prince, who is nowhere more at home than astride a gym horse, having "noble thoughts" for the benefit of his portraitist.

The play's monumental structure is riven with flaws, but even its imperfections have something to say. In a clumsily integrated break with period, a modern doctor informs George's physicians that the king was not mad but suffering from a metabolic disorder: different age, different label. In another scene, George Savours King Lear, while insisting that its tragic ending is a mistake. Obliquely, as is his wont, Bnnett reveals himself: his gleeful dismissal of orthodoxy, and his refusal to follow his pessimism through to its logical conclusion - in this case the imminent relapse of the good king into madness.