A Voyage Round John Mortimer, By Valerie Grove

Sir John Mortimer's status as a national treasure has come in for a bit of battering in recent years. A hostile biography by Graham Lord, in 2005, revealed Mortimer to have had an extramarital affair at the beginning of the 1960s with that epitome of sitcom cosiness, Wendy Craig – and that, furthermore, the relationship had resulted in the birth of a son, Ross, who had been brought up by Craig's husband Jack Bentley as his own.

Predictably, this secret-love-child saga proved irresistible to reporters and columnists, and not just those writing for tabloids. "Rumpy-Pumpy of the Bailey" ran one broadsheet headline, a reference to Horace Rumpole, Mortimer's most famous fictional creation. Further revelations about this latterday Lothario – or perhaps one should say Falstaff – were soon forthcoming. Molly Parkin went public about her affair with Mortimer, and talked about his predilection for being spanked with a hairbrush. Although Mortimer has since claimed that Lord unwittingly did him a service by reuniting him with his long-lost son, subsequent disclosures have thrown doubt on his claim that he knew nothing of the baby's existence at the time of his birth.

Valerie Grove's new biography of John Mortimer bears the label "authorised", though in fact Grove is no pushover as a biographer, and certainly doesn't let her subject off lightly. She has faced several disadvantages in writing this book, not least that, at 84, John Mortimer, after a staggeringly energetic personal and professional life encompassing a career as a barrister as well as a novelist and playwright, is still very much alive. She has had no extensive archive of letters and diaries to work from either: out of his legion of friends, she says, not one of them could show her a single letter from him since 1944. She had quickly to get to grips with Mortimer's own ambivalent relationship with the truth. He has plainly always possessed a gift for factual inaccuracy. He has told, and retold, aspects of his life story – about his marriage to the writer Penelope Mortimer and his relationship with his barrister father, Clifford – so many times in novels and plays that even he himself now finds it impossible to differentiate between what is true and what is fiction.

However, Grove has managed to surmount these difficulties. She knows how to create an engaging narrative from the patchwork of interviews and background material she's collected, and triumphantly gives us a portrait of the man, warts and all. Grove leaves us in no doubt that John Mortimer is an immensely talented and monumentally selfish individual, whom it's nonetheless almost impossible to dislike.

Grove's revelations about John's passionate friendship with a younger boy, while he was up at Oxford, will certainly discomfort the Middle England Mail readers who, one suspects, form Mortimer's natural constituency as an author. She vividly recreates the figure of Clifford Mortimer, blinded in an accident when John was about 12, who became the subject of Mortimer's most famous play, A Voyage Round My Father, in which Clifford's expiring words, "I'm always angry when I'm dying," have been declaimed by a series of actors in the role, from Alec Guinness in the original, to Laurence Olivier in the TV version, to Derek Jacobi in last year's stage revival.

Undoubtedly Grove's greatest achievement lies in her portrayal of Mortimer's first marriage to Penelope Mortimer. Penelope was beautiful and dangerous, holding up to the light the break-down of her marriage to John in the scabrous prose of novels such as The Pumpkin Eater (which, though remarkable in its time, reads today like a faded period piece). Their literary rivalry, her mental fragility, a succession of break-ups and reconciliations, are brought horribly to life. In 1971 Mortimer met his second wife, also called Penelope but known as Penny, who emerges as the most likeable character in Grove's book. Capable and unflustered by celebrity, the second Mrs Mortimer suffered the indignity of being lampooned by the first. In her novel The Home, Penelope Mortimer included a venomous pen-portrait of her successor as Nell Partwhistle, "who sat about all day brushing her hair, and couldn't even cook cheese on toast".

John Mortimer has been a champion of free speech as a defence barrister in a number of celebrated cases, including the Oz trial in the 1960s, and the Gay News blasphemy trial in the 1970s. He has written many bestselling books, seen his name in lights in the West End and had a hand in innumerable screenplays, but still regrets having never written a great novel. "I straddle the chasm," he once said, "between Jeffrey Archer and Salman Rushdie." He remains elusive: it will always be a case of a voyage around John Mortimer rather than a voyage to his centre. But on the evidence of this book, it's been a rollicking life, and one that Grove tells with vitality, balance and humour.