Smaller, Lyric Shaftesbury Avenue, London <!-- none onestar twostar threestar fourstar fivestar -->

Smaller seems a somewhat ironic title for a play that has been made to measure for Dawn French and Alison Moyet. But size is not what really matters in this piece - a first foray into stage drama by Carmel Morgan, who has written cracking scripts for Coronation Street and Shameless.

The play came about because French and Moyet, who are friends, wanted to be in a show together. They make convincing sisters, but for most of the proceedings they operate in separate worlds.

French plays Bernice, a middle-aged teacher who has spent the past 25 years tending to the many needs of their widowed and disabled mother, Maureen - an exasperating chatterbox and devout Catholic, beautifully portrayed by June Watson. Moyet is Cath, the tougher daughter who flew the nest and was once a promising singer. Now, though, she is reduced to belting out karaoke classics in preposterous costumes for tacky tourist groups on what Bernice tartly calls "the Costa del Chlamydia". While Cath, clad in a chicken outfit, is massacring a Robbie Williams medley, Bernice is heaving her mother from wheelchair to loo, wiping her bottom and pushing her piles back in.

The minefield of resentment, guilt and sorely tried dedication that surrounds the relationship between sole carer and helpless parent is not uncharted territory; nor is the resulting acrimony among siblings. But Smaller approaches them with an attractive mix of robust humour, honesty and humanity. In the first half, it cuts too often to Cath's humiliating performances in Puerto Banus - and the numbers that Moyet has written don't integrate well with the spoken drama.

But in Kathy Burke's affecting production, French and Watson capture the stifling co-dependency of the trapped couple. French is very funny, conveying the strategies of broad sarcasm and comic send-up that stop the daughter from going insane. But she also transmits an underlying patience, good humour and love.

It's Bernice who feels the stronger guilt after Maureen's death. "If I'd had less of Mum, I would have had more of her," she proclaims in the rancorous recriminations with her sister toward the end. The woman who had resented the daily debriefings from her mother is left with the sad, angry feeling that she has much still to tell her.

Booking to 6 May (0870 890 1107)