The Last of the Duchess, Hampstead Theatre

Is it nostalgic to constantly revisit the history of the royal family? In this new play by Nicholas Wright, which opened last night, we travel back in time to 1980 when the aged Wallis Simpson - widow of the abdicated King Edward VIII - lived as a recluse in a mansion in the Bois de Boulogne, Paris. Despite the fact that a national treasure (Sheila Hancock) is starring in the play, is this a subject worth looking at again?

OK, the story is intrinsically interesting. After the death of Edward in 1972, the Duchess lived in splendid isolation. Then, in 1980, The Sunday Times planned to publish a profile of her by Lady Caroline Blackwood, a society writer. But when she arrives, she soon finds her access to the reclusive royal blocked by the octogenarian Maître Suzanne Blum (Hancock), the Duchess’s lawyer, friend and general bloody-minded control freak.

Caroline is gradually drawn into this strange spider’s nest of a household

In fact, Caroline soon finds herself intrigued by the character of Blum, a much more interesting person than the sad and bedridden Duchess. In particular, she begins to ask questions about why Blum keeps the royal out of sight: could she be dead? Caroline is gradually drawn into this strange spider’s nest of a household, with Blum spinning her webs of deceit, while her retinue – Michael Bloch, a young lawyer, plus an impassive butler and a housekeeper – scuttle about like inferior insects.

But Blum’s interest for Caroline comes from the fact that she seems to be keeping a secret. What is the nature of her stranglehold over the octogenarian Duchess? Other secrets gather in the dark corners of the gloomy mansion. One of her few remaining friends, the loyal Lady Diana Mosley (Angela Thorne), talks about the old lady’s jewels, which keep cropping up for anonymous sale. But who is benefiting from the money?

Based firmly on Caroline’s own book, also called The Last of the Duchess, Wright’s account is marvellously light on its feet, effortlessly conveying the story, conjuring up the eccentricities of life at the edge of celebrity. As an image of old age, decay and vanity, it is moving. And the themes of snobbery, secrecy and sex are refracted through the cracked champagne glass whose crystal throws suggestions of corruption and decay like eerie lights over the gloomy mansion.

The programme’s cover shows a photograph of the aged Duchess taken through a window: on the ledge, a pigeon stares inside. At times, the play has a similar distancing effect: we are that pigeon, outside gazing in. And as we peck at scraps of gossip we never really get to see the Duchess, but instead have to content ourselves with the eldritch hilarity of Lady Mosley, as deaf as a gilded doorpost, chatting with a sloshed Caroline.

Wright brilliantly suggests that, despite the deepest research, we can never, ever really get to know other people. Like everyone else, journalists create fictions out of messy reality, projecting their fantasies onto their subjects. So Blum’s airbrushed version of Wallis’s life is contrasted with Caroline’s much more scandalous account. In the struggle between the two, truth begins to recede and the mystery deepens.

One thing is for sure: this account of the royal family is a welcome counterpoint to the uncritical coverage of their relatives’ heroism in the Second World War or their glamour in more recent wedding celebrations. Wright’s play reminds us that Edward VIII and much of the aristocracy in the 1930s were crypto-fascists and soft on Hitler. Lady Mosley is a cheerful anti-Semite. Not very nice.

Directed with enormous sympathy by Richard Eyre, the acting does not disappoint: Hancock radiates authority and suggests a yearning for youth. Her voice rises and falls like the temperature of an invalid, sometimes coolly deep, sometimes rising to a shrill wail. As Caroline, Anna Chancellor (pictured above right) - who stole the show in The Hour - is great in her portrait of a woman on the skids, her familiar contralto instantly recognisable. This is a witty, thought-provoking and rather elegant entertainment. Yes, it was well worth resurrecting the ghastly Windsors.

• The Last of the Duchess at the Hampstead Theatre until 26 November