You wait years for a drama about Wallis Simpson and then three turn up at once. Last Christmas, she popped into 65 Eaton Place during an episode of Upstairs Downstairs and, this year, she got a biopic of her own – albeit somewhat derided – courtesy of Madonna.

Now here she is in the opening dream sequence of Nicholas Wright’s latest play, leaning louchely against the mantelpiece of her Boulogne chateaux and fixing herself up with small buckets of ‘vawd-ca.’

That is indeed the last we see of the Duchess. For the rest of the play, adapted from Lady Caroline Blackwood’s book of the same name, she is bedridden upstairs, rumoured to be senile, shrivelled and mute. Possibly even dead.

In April 1980, Blackwood was dispatched by the Sunday Times to profile the Duchess of Windsor, only to be denied access by her lawyer and protector Maitre Suzanne Blum (Sheila Hancock, outmoded and frosty as granita). Instead, with Blackwood sniffing around for a scandalous scoop, Blum herself becomes the piece’s subject and faces accusations of theft and abusing her power of attorney.

That sets up a rather fascinating game of cat and mouse between interviewer and subject, though it takes a long time to get there. Wright’s first act, teeming with high-society tittle-tattle, is like a staged edition of a vintage Tatler. It lacks the double perspective to mine universals from its aristocratic subjects.

However, after the interval, Wright settles down to business proper and presents a proper journalistic duel. While Blackwood, joyfully played by Anna Chancellor with the lolloping surliness of a tipsy teenager, builds towards a cry of "J’accuse", Blum guards the Duchess with parries and deflection. Wright makes an entertaining and even bout between the ruthless and the rueful.

Beneath all this is the question of truth and representation. With both Blackwood and Blum’s versions skewed by their opposing motives, Wright’s concern is with history’s gatekeepers. As the piece ends on a mournful note, he sides with neither Blum’s self-elected censor nor Blackwood’s bitter megalomaniac. Nor, in an admirably neutral piece, is he naïve enough to advocate unbiased truth above all else.

Chancellor and Hancock make worthy adversaries, each filling their role with characterful forthrightness, but Richard Eyre’s production would be better served by a less literal staging. Though Anthony Ward’s copper green gauze walls add a ghostly quality, the naturalistic setting – all regency sofas and antique statuettes – emphasises Wright’s light drawing-room comedy over its titanic clash. It does, however, allow decent comic turns from John Heffernan and Angela Thorne as Michael Bloch, Maitre Blum’s own loyal protector, and Lady Moseley respectively.