London Assurance, NT: Olivier, London

Simon Russell Beale made his reputation, over 20 years ago, playing egregious Restoration fops at the RSC. He now returns, in a blaze of comic glory, to this effeminate breed in Nicholas Hytner's deliriously funny revival of Dion Boucicault's 1841 play London Assurance.

As Sir Harcourt Courtly, Bart, Russell Beale plays a mid 19th-century relic of the Restoration fop. Fifty seven and sporting dyed-brown kiss curls, this deluded sprig of the nobility has decided to shore up his fortunes by wooing a heroine roughly a third of his age. This involves a trip to the country which, for Sir Harcourt, is about as congenial as a voyage to Saturn. Once there, though, his attentions are distracted by the uber-equestrian Lady Gay Spanker. She sounds like something you would encounter on a card in a urine-scented Soho telephone booth; in fact, she's like a cross between Jorrocks and George Sand, a clever woman who thinks entirely through the metaphor of the hunt with which she is crankily obsessed. She's played here, in a performance of serenely mad, side-splitting verve and finesse by Fiona Shaw.

Theatre is nothing if not a lottery and fans of this pair may think it odd that we have waited half a lifetime to see them together, only to find them in what the snobbish might regard as a comedy at the low-rent end of the repertoire. To which the riposte is two-fold. They strike hysterical sparks off one another – and Hytner, who seems to have done a bit of adroit script-doctoring with playwright Richard Bean, makes a very persuasive case for the sharpness of this comedy. Russell Beale and Shaw, as actors, are a partnership made somewhere decidedly north of Purgatory.

He brings to the table all his brilliant talent for embodying fops: the goggle-eyed, faint-hearted vanity; the sense of an IQ slightly lower, numerically, than the spherical waist-measurement; the pampered, plosive vocal delivery that seems to "goose" the lines slightly gingerly, especially when they are in affected French. Shaw makes one of the funniest entries into a production that I have ever seen. Lady Gay has galloped over hill and dale to get there and, absolutely beside herself with wordless, horsey mirth, she relives the entire journey in a convivially potty, head-tilting mime.

To 2 June (020 7452 3000)