First Night: London Assurance, Olivier Theatre, National Theatre, London

Sparks fly in an assured comic display

Simon Russell Beale made his reputation, over twenty years ago, playing egregious Restoration fops at the Royal Shakespeare Company. He now returns, in a blaze of comic glory, to this effeminate breed in Nicholas Hytner's deliriously funny revival of London Assurance. This is the play with which Dion Boucicault, the young, gifted, and dodgy Irishman, future bigamist and gainer and loser of fortunes earned his spurs on the London stage at the age of 21 in 1841.

As Sir Harcourt Courtly, Bart., Russell Beale plays a mid-nineteenth 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 county 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. If you dispense with the honorific, 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 not, say, playing Beatrice and Benedick, but 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. In his expert hands, the play brims with brio and youthful high spirits. To use an oenophile metaphor: it's not like savouring a full-bodied, venerable Burgundy; it's more like getting deliciously tiddly on bumper after bumper of Beujolais Nouveau.

Sir Harcourt and Lady Gay are about as martially compatible as – ooh – Christopher Biggins and Princess Anne. But Russell Beale and Fiona 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. If she were to wander into the production of War Horse, you feel that the stars of that show would welcome her as the missing link between them and, say, My Friend Flicka. Or Black Beauty.

The performances are first rate across the board. As Lady Gay's doddery husband, Richard Briers is amusingly given to involuntary shudders that sometimes accidentally seem to be protests at Sir Harcourt's preening recourse to French. Michelle Terry brings a lovely sulky intelligence to the realistic firmness of the young heroine Grace and Nick Sampson excels as the kind of valet who is more fastidiously snooty than his "betters".

Part of the joke is that the "country" is evoked with a mischievous approximateness. On two occasions, a large rat scurries onstage – Simon Russell Beale's Sir Courtly reacts by thrusting his leg forward even further, as though maximising his effeteness will somehow be a repellent. Bliss.