Forman: It's my life and I like it

What exactly is a clear water worm? Sir Denis Forman leant forward in his red braces and raised a glittering eye. "Now that," he said, "is a really interesting question." Abandoning a lukewarm discussion of Scottish devolution, he took a pensive puff at his pipe. "Around this time of year, all the water in the streams and rivers of Scotland is as clear as gin. It is exceedingly difficult to entice a trout because he can see everything. So, what you do is, you take a worm ..." he began to explain.

Sir Denis has been many things in his long life, among them director of the British Film Institute, deputy-chairman of the Royal Opera House and, most famously, chairman of Granada Television during its glory days. He is revered in the ratings-wary, accountancy-minded, post-Birtian broadcasting world as the man who presided over the creation of The Jewel in the Crown, World in Action and Coronation Street. Now he has notched up another singular achievement: his own boyhood has become the subject of a full-length feature film.

Imagine being able to trot around the corner to your local cinema and watch yourself winningly depicted catching your first trout in a movie directed by Hugh Hudson, produced by David Puttnam and starring Colin Firth as your father. Sir Denis seems modestly delighted that with My Life So Far playing at the Hampstead ABC, only a few steps away from his garden flat, he can do just that. Well, not trot, exactly: he lost a leg at the battle of Monte Cassino in 1944, which ended his days as a tireless Scottish reeler. But he remains a remarkably hale-looking 82 and still goes loch fishing every year. He has been to see the film twice, applying a professional eye. He finds it "highly agreeable", graciously passing over the implausible fictional embellishments.

My Life So Far is based on Sir Denis's funny, touching and evocative memoir of his childhood in the Dumfries countryside in the 1920s, when farms were still run on horsepower and before the scythe gave way to the combine harvester. The title of the book, published 10 years ago, was Son of Adam, and the author rather wishes the film-makers hadn't changed it. "I think mine was better." Adam was the name of his father, the decent, God-fearing but deeply eccentric figure who became the focus of Denis's pubescent rebelliousness.

The Rev Adam Forman, head boy of Loretto and later its chaplain, never shed his bracing, public-school attitude to manliness and godliness, even after he married and became the factor, or agent, on his mother-in-law's estate, Craigielands. When he wasn't holding Presbyterian discourse at the dinner table or herding his large family off to St Mary's United Free Church for their spiritual improvement, he was seeing to their physical cleanliness with year-round bathing in the loch, except when the ice was too thick. His small sons were intrigued to discover he kept a morsel of cotton wool "about the size of a pea" inside his foreskin for purposes of personal hygiene. It was in keeping that Adam's most notable achievement was the harvesting of quantities of sphagnum moss during the First World War, to be processed into field dressings for the wounded. He claimed its properties far exceeded those of cotton wool. For this unusual wartime initiative he was awarded the CBE.

The smell of moss was not the only one to imprint itself on Denis's young memory. He and his five brothers and sisters (Denis, the chief mischiefmaker, was number four) were horribly perceptive in the olfactory department. The visiting seamstress smelt of exhaust fumes combined with "the inside of a spaniel's ear and dead rat". Marnie the housekeeper ponged of unwashed underparts and embrocation, with overtones from her wig of "dead mouse blended with nutmeg".

Much of this comically perceptive writing is lost in the film, so we never properly encounter the highly strung cook, Mrs Henderson, who "at times of great stress became hysterical, throwing her apron over her head and making noises like a railway engine taking in water".

"But we were not a posh household, with liveried foot-men," Sir Denis insists. Passage through the green baize door was free and easy. His intimacy with the servants, his liking for working in the fields alongside the farm men at harvest time and the camaraderie of the great curling contests on the frozen loch form a key theme in the book, leading the little boy to think that "perhaps I preferred the likes of them to the likes of us". Even before he is sent away to boarding school he is kicking against the pricks of class and religion, spending furtive hours reading up the entry under "Prostitution" in the Encyclopaedia of Ethics.

Wasn't it odd that those early seeds of rebellion should have led to a career spent entirely among the officer class, from being head boy of Loretto onwards? Not at all, says Sir Denis. "I was always subversive." Even in the army he "aided and abetted" the shift of thinking that led to so many servicemen voting Labour after the war.

In his television years he was a determined advocate of investigative journalism. Today he deplores TV's lack of will to explore a controversial subject in depth. The Zimbabwe Test match was under way. "Who's going to do corruption in cricket?" he asked rhetorically, knowing the moment has passed.