A new bout of Brideshead mania seems set to engulf the nation with the imminent release of a big-budget film version of Evelyn Waugh's classic novel. Before it does, one matter needs to be settled very firmly.

Rows have already broken out about how far Hollywood has deviated from the book. The Daily Telegraph reported on Saturday: "Sir John Mortimer, who remained faithful to the original text in adapting it for the 1981 television series, is among those who have criticised the new series."

Now Brideshead Revisited is a subject over which, I would have thought, Sir John would be wise to preserve a discreet silence.

However unpalatable it might be to him and his many loyal friends, the truth is that Mortimer was not the literary force behind Granada's award-winning drama. Though his name appears on the credits, none of the words spoken by Jeremy Irons, Anthony Andrews, John Gielgud, Laurence Olivier and the rest of the distinguished cast was written by Mortimer.

He supplied a script, to be sure, but this was rejected by producer Derek Granger and the first director Michael Lindsay-Hogg. They decided instead to "shoot the book", using Waugh's exact words, a process that continued when Charles Sturridge took over direction.

The story of this deception on the viewing public - which is what it amounts to - was told three years ago in Graham Lord's unauthorised biography John Mortimer: The Devil's Advocate. It caused many a hissy fit from Sir John's luvvy pals, but was confirmed in every detail last year in Valerie Grove's authorised life.

She wrote: "Although John was handsomely recompensed . . . consider how awkward it was for him to be on the receiving end of the plaudits, in perpetuity, for a work which in style or substance could have been his, but in fact was not."