There’s a certain strain of privileged home-county characters who often appear in plays: blustering, petty and priggish, easy to laugh at, and difficult to empathise with. What usually follows is a comedy about suburban values and middle-class pretensions, in which the whole charade goes up in flames. Alan Ayckbourn’s Woman in Mind at first seems destined to follow this trope, but he takes this fairly ordinary story and creates a far more interesting study of reality and fantasy, which takes the audience along on its decent into madness on the back of howls of laughter.

When we first meet Susan (Bethan Davidson) she is flat out on the grass of her garden, beside the offending rake, which has knocked her unconscious. She’s being fussed over by a flustered doctor (Helen Vella Taylor), apparently speaking garbled nonsense. Fortunately, in a blaze of green light, Susan’s gorgeous and fabulous family – her daughter (Hollie Witton), husband (Xanthe Burdett) and brother (Carine Valarche) – clad in immaculate white, emerge and come to offer sympathy.

Alas, this family turns out to be nothing but a fantasy, and her real family consist of a passionless buffoon of a husband, played with wonderful self-importance by Benedict Flett, a pathetic and patronising sister-in-law (Louisa Keight) and a prudish son (Joe Pieri), who has joined a philosophical cult and has ceased talking to her. She’s unfulfilled, unsatisfied and unnoticed.

As the play progresses, the real and imaginary families become mixed up, and the latter starts to take on a life of its own. Even her imaginary family come to disappoint her, apparently failing to meet the achievements of those in her real world, and reminding her of her own lack of achievement. The thin veneer of cosy family rituals wears thin and eventually falls away completely, leaving exposed an unsettling and hysterical vision of mental torment and bitter disappointment with life.

All the cast inject their ridiculous one-dimensional characters with enough hilarity to be bearable, but they are all ultimately pillars to support Susan. This is a one-woman show, and Davidson delivers a performance that conveys savagery and charm. It’s a performance that is as much in her suppressed expressions as in the delivery of the lines itself. Her marriage is miserable, and her own doctor would rather talk to her husband about her condition than her, but she allows you to constantly catch sights of the woman Susan used to be: witty, sensual and cutting.

The thrill of the play as it descends from a portrait of unease into tragedy is seemingly easy in this production, and the climatic moments capture the full poignancy of Susan left alone in the dark, seemingly incomprehensible and forever ignored.