Jane Austen has probably overtaken him as the nation's favourite old-time novelist, but Bos can still pull a crowd. Simon Gray's investigation of the great man's extra-marital activities is enjoyable enough to please enthusiasts, but pulls too many punches to enthral the uncommitted.

The Nell of the title is neither the Dickens character nor the inspiration for her. Nelly Ternan served as Dickens's mistress for the last 13 years of his life. She was 17 at the outset, he 45. Gray frames the affair by showing us a later encounter between Dickens's son Henry (Barry Stanton) and Nell's son by her later marriage (Tim Pigott-Smith), who spends most of the play shaking his head at the shock of it all.

For Michael Pennington's Dickens could be seen as a full-on cad. Though he plays the lovesick sentimentalist, he woos the naïve Nell ruthlessly and with twee condescension, whisking her off to the country and inviting her to play with his beard.

Yet Gray is never bold enough in his iconoclasm. At every turn, Dickens is excused. Henry recounts how much Dickens loved Nell, how much his children liked her, how comparatively pleasant was her subsequent life. The apple-cart remains upright.

Lou Brealey covers Nell's progress from girlish bachelor bait to prim, suburban motherhood well, yet her darker moments - the frustration of being written from Dickens's history, the revulsion at serving a decrepit lover, the despair of giving away their children - are merely touched upon in passing, along with Dickens's gout and favourite readings. Gray seems more interested in giving us all the facts than tapping any rawer emotions.

The result is something that is rarely boring, but more documentary than play. Gray has let moral complexity get in the way of a good story, something his subject would never have done.