There is a ridiculous but characteristic moment in Amy Rosenthal's psychologically naive play about DH Lawrence's disappointments when he tried to found a ménage à quatre on the Cornish coast in 1916: the famous writer flourishes a carrot in front of his bemused friend, literary critic John Middleton Murry, who to Lawrence's disappointment fails to identify the vegetable as either sex symbol or phallic object.

This incident encapsulates the failure of On the Rocks. Miss Rosenthal has created an old-fashioned marriage comedy from material quite resistant to such colouring or simplification. Lawrence's erotic longing for Murry, a desire whose nature he misrepresented to himself and which helped inspired his great novel Women in Love, caused both men's partners unhappiness and jealousy.

In 13, not well co-ordinated scenes Miss Rosenthal traces the course of a failed experiment in living when Murry and his lover, New Zealand short-story writer, Katherine Mansfield, come to live next door to Lawrence and his German wife Frieda.

Designer Paul Burgess conjures up two, abutting, oddly unlife-like cottages. Charlotte Emmerson's excellent Katherine, all bleak, tense watchfulness at her upstairs desk, loathes the isolated environment, finds herself unable to write and then neglected by Murry, who goes off for long, daily walks with Lawrence.

Miss Rosenthal never captures the sadness of this sublimated, unacknowledged passion, nor does she catch its conflicted complexity. On one crucial occasion Lawrence strips naked and wants to persuade Nick Caldecott's Murry, who removes his three-piece suit and displays a thick layer of embarrassment, to wrestle with him. In another, while the two men lay a carpet, Lawrence tries to interest Murry in an odd rite of blood-brotherhood that involves self-harming.

Both scenes, in a production by Clare Lizzimore that lays undue stress on jocularity, render the two men comically absurd, even though Ed Stoppard makes a fine, convincing Lawrence, his volatile moods and rages apparent symptoms of emotional disturbance.

But Caldecott, miscast as Murry, bears the trappings of a prim vicar, transported from a Ben Travers farce. Lawrence's relations with Frieda, in real life clearly sado-masochistic and hampered by impotence, are here simplified into outbursts of physical abuse, followed by unbelievable bouts of sexual abandon. Tracy-Ann Oberman's Frieda, speaking in an often unintelligible German accent, revels in aggressive self-pity - another case of Rosenthal's simplifications.

Until 26 July (020 7722 9301)