Peter Shaffer wrote what he called this "very English piece" in 1987 for Maggie Smith and in 1990 both her Lettice and Margaret Tyzack's Lotte won Tonys on Broadway.

Lettice Douffet is as eccentric and exotic as her name. An actress manqué working as a tour guide in a frankly dull stately home, she comes to revel in inventing ever more thrilling anecdotes involving the grand staircase and a visit by Elizabeth I to sex up her tours, to the increasingly delighted interest of most of her visitors.

Unfortunately for her, more informed visitors are outraged by her embroideries of the truth and it's only a matter of time before she meets her apparent nemesis, Lotte Schoen, manager at the Trust that owns the wonderfully-named Fustian House. But Lettice's sense of theatre, inherited like her name from her French actress mother, ensures that she turns her dismissal into a coup de théatre and marks the beginning of an unlikely friendship between the two.

Eccentric Lettice may be, but it's her passion and bravery that attract the apparently more conventional Lotte and eventually they discover a shared hatred of the "mere" as exemplified in brutal modernist architecture - and a shared love of a rather extreme form of historical reenactment that almost has disastrous consequences for life, limb and their friendship.

Shaffer's artful writing is an extraordinary duet for two women and proves a marvellous vehicle for Selina Cadell and Jessica Turner. Cadell succeeds magnificently in making Lettice her own and Turner's Lotte is younger and more vulnerable than the indomitable Tyzack's. Shaffer's dialogue, or often monologue, is a series of sweeping cadenzas and riffs which could meander in the wrong hands, but here sparkles and surprises at almost every turn under Matthew Lloyd's capable direction.

It's truly touching to watch the developing relationship between two lonely women of a certain age. So that when it looks like the game is up and there's genuine fear that those extreme reenactments may land Lettice in jail and lead to the pair's estrangement, you feel for them both.

There's fine support from Michael Thomas as the solicitor charged with providing legal aid to Lettice. His face is a study of ever-increasing incredulity as Lettice acts out the incident that apparently almost led to Lotte's death by beheading, and the way he becomes swept up in her tale is genuinely liberating. Helen Mallon makes a sympathetic downtrodden secretary to Lotte and a posse of volunteers do sterling service as visitors to Fustian House.

Designers rise to the challenge of the intimate Watermill in extraordinary ways, and Andrew D Edwards counts among the cleverest. His wood panelling makes Fustian Hall and Lotte's office; and he brilliantly opens and turns the panels to reveal Lettice's bohemian Earls Court basement, delightfully reminiscent of circus caravan interiors I've visited and complete with stairs vital to the plot. Even the theatre's gallery plays its part, echoing with the footsteps of Lettice's visitors. A richly rewarding evening.

- Judi Herman