I worried if David Hare's 1995 altercation between ex-lovers, a restaurant entrepreneur and a do-gooding East End schoolteacher - played, respectively, with buccaneering dash and quiet fervour by Bill Nighy and Carey Mulligan - might have dated. And the flagrantly anachronistic orange Sainsbury's shopping bags in the first scene were a bad omen.

But it really hasn't. Nighy's designer-suited Tom Sergeant, who has sold his business to a corporation in the spirit of "the creation of wealth," simply doesn't understand that a dedicated teacher's delight in finding one under-privileged student who responds is worth a hundred good dinners, let alone a heating system that actually works.

Having soaked up Tom's hysteria, guilt and desperation for several hours, during which they renew their carnal vows, Mulligan's Kyra delivers two long speeches with such devastating simplicity and passion that the first of them shocked a first night audience into semi-reluctant applause.

The brilliance of the play lies in the fact that the arguments are not loaded yet you know who's right all along. This is because the debate is artfully bound up in a re-cap of a six-year love affair between a man whose wife has died and his employee - Kyra walked down the King's Road one day and found herself a job as a waitress - and they all lived under one roof.


"I've seen both previous London productions... and this is probably the best"

Kyra baby-sat the children, one of whom, 18 year-old Edward (a gangly, oddly eccentric Matthew Beard with a rap CD player on his belt, and earphones), acts first as an intermediary, later as a friend. And the nub of Hare's construct is that it's as important to love one person as it is to love people; or rather, both are equally hard.

Stephen Daldry's savagely intense production delivers the knottiness and the sentimentality of the idea full blast on a set designed by Bob Crowley that places the clutter of Kyra's Kensal Rise flat against a regimented vista of a grim council block and a few sprigs of nominal trees, bare in winter. There's snow, too.

I've seen both previous London productions - Richard Eyre directed first Michael Gambon with Lia Williams at the National, then Nighy with Stella Gonet at the Vaudeville - and this is probably the best. It's more urgent, more musically enthralling, and Nighy has gone to another level, a sort of stylised self-conscious jerkiness that fits the character exactly, his vanity and his slashing selfishness, both of which have been seriously undermined by Kyra.

It's fascinating to watch because it's so unusual, and curiously physical, a performance. The coat he refuses to take off in the first act fits him like a second skin (Gambon's was like a big furry tent, turning him into a mountain bear). It's the finest outer garment in the West End since Madonna's Prada raincoat in a David Williamson play, Up for Grabs, on this same stage in 2002. And Mulligan soaks up his oddness like a sponge before dealing her knock-out blows.

Hare wrote this play - which is broadcast as part of National Theatre Live on 17 July to over 500 UK cinemas - two years after his great state-of-the-nation trilogy on the church, the law and New Labour at the National, and this could have been regarded as a coda; but it's much more than that, in both its analysis of a love affair and in its well-crafted shout-out for the deserving, still undervalued (and underpaid), members of our society who do the most important work.

Skylight continues until 23 August