Not long ago, the author of this work, a former books editor of the Sunday Express, installed himself on my sofa and invited me to tell him all about my affair with Sir John Mortimer. I had never had such an affair; indeed, I could not remember ever having shared Mortimer's delightful company with any fewer than 50 of our closest friends, so I found myself denying the accusation with unchivalrous vehemence.

The conversation then languished. I had nothing to say about Mortimer, other than that I had always found him to be charming, witty, clever, talented, generous, industrious, warm-hearted, sensible, touchingly vulnerable, indulgent to children and adoring of his beautiful wife. This was clearly not what the man on the sofa wanted to hear. Why, I wondered, had he chosen to write about someone he didn't like?

Perhaps because he thought it was going to be easy. As the most famous liberal QC of his generation, Mortimer appeared in several well-reported landmark cases, including the prosecutions of the underground magazine Oz, Gay News and Hubert Selby's novel Last Exit to Brooklyn. In his 60-year literary career, he has written an autobiography, three volumes of memoirs and several autobiographical novels, plus his masterpiece, A Voyage Round My Father, an autobiographical play that was made into an award-winning television drama. His first wife, Penelope, also wrote an autobiography and a clutch of autobiographical novels. Much of the research for this book must have been like shooting fish in a barrel.

A few phone calls cleared up any remaining mystery. Lord had proposed the work to Mortimer and had done some preliminary interviews. His subject then noticed that this scribe was something less than the Boswell of our times, and so withdrew his co-operation. Then, in danger of becoming the defenceless victim of a hatchet job, Mortimer adopted Valerie Grove, a far more distinguished author, as his official biographer. So what we have in this volume is the hellish fury of a biographer scorned.

To call this book catty would be an insult to cats. It amounts to 300 relentlessly spiteful pages of proof that you can take a hack out of the Sunday Express, but you can't take the Sunday Express out of the hack. There are the attitudes: as far as Lord is concerned, liberal, radical, socialist, pacifist and feminist are all terms of abuse. There is the language: utilitarian, vulgar and ugly. There is the focus: forget the law, forget literature, forget politics, forget the friendships with the best and brightest - what Express readers want is sex.

Despite Lord's determination to depict his subject as a serial adulterer, he has found barely a handful of women to admit to affairs with Mortimer, two of whom Horace Rumpole, the Old Bailey survivor who is Mortimer's best-loved fictional character, might describe as notorious doxies. Three of them stress that the liaison was insignificant; one would certainly admit to being an enthusiastic drunk.

Otherwise, the bulk of evidence is culled from the writings of the first Mrs Mortimer, who had a history of serious mental illness. Rumpole might well submit that, in the Casanova stakes, this leaves our man trailing the field. Most of the author's other propositions are equally dubious, and it is with frequent mutters of "So what?" or "And that's a bad thing . . . why?" that the reader turns the page.

The spirit of Mortimer's times is igno-red. Much of the alleged action took place in the 1960s and 1970s, when free love was fashionable and "trendsetters" routinely claimed to be rutting like crazed weasels, in the words of another contemporary playwright, Anthony Shaffer. More importantly, Mortimer's legal career began in the 1940s/1950s, when the profession was a bastion of prejudice, snobbery and repression.

Back then, in the days of capital punishment, lawyers were nearly all men who slavered over the sexual gossip culled from their divorce cases, while upholding a system that dismissed rape, domestic violence and paedophilia, and criminalised homosexuality. The liberalism at which Lord sneers has created a better system of jus-tice and a more open and compassionate society. Mortimer played an important role in the process, and there is a great biography of him still to be written. Let us hope that Valerie Grove is hard at work.

Celia Brayfield's most recent novel is Wild Weekend (Time Warner Books)