If you thought Susan Hampshire was just a lightweight actress whose speciality was to exude charm, think again.

At the Theatre Royal this week, she has become an unsmiling, mad old lady who rants on about life, religion and her ambition to become Prime Minister.

She sports grey, matted hair, layers of filthy clothes and seems not to have washed her face in months.

She is one of London's mad derelicts who somehow ended up living in an old Bedford van in Alan Bennett's driveway.

The play is based on two events which happened in the Seventies and Eighties and it takes two Alan Bennetts to tell her story.

Hampshire - once Fleur Forsyte in the original television version of The Forsyte Saga and currently Scottish matriarch Molly in The Monarch Of The Glen - gives a tour de force performance as the mad Miss Shepherd, who got Bennett's creative juices flowing in to a diary, a book, a radio serialisation and this play.

She is mesmerisingly cantankerous in the role as she makes her querulous demands known.

The two Alan Bennetts are a superb piece of theatre. One, played by David Holt, is the writer Bennett, coolly using the experience to make art. The second, Paul Bigley, is Bennett as carer - the man actually experiencing the situation and being drawn into looking after the woman.

It is a clever ploy and, although disconcerting at first, it soon becomes clear what is happening. And like all Bennett's work, it reveals much about him, much more than any interview this most modest of modern playwrights will give.

We learn of Bennett's growing up, his relationship with his mother and the things which drive him on.

This could be his most intimate and revelatory play. Both actors have his distinctive accent and both look enough like him to give added reality to this tragicomedy.

But this is essentially Miss Hampshire's story. Through her, we venture into a dark and disturbing world. Buried within her is a former pianist, a former nun and a good mind.

Bennett's use of language is as applicable and highly literate as ever. His references to his wide-ranging reading are richly complex, yet not once does he talk down to his audience.

This is a work from a major talent who has been amusing us ever since Beyond The Fringe tickled our funny bones in the Sixties.