This is in no small measure due to Bradley's naturalistic, BAFTA-winning performance as Billy, which ranks alongside those of Jean-Pierre Leaud in Truffaut's Les Quatre Cent Coups and Henry Thomas in ET, as the best leading performances by children I can think of. Without his repressed energy, sullen innocence and flashes of comic insolence, Kes may have been unbearably drab and depressing.
So it seems sad and strange that, at 44, the grown-up Bradley is virtually unknown. He has done some acting work since Kes - most notably a stint with Peter Hall's company at the Old Vic - but today he's unemployed and struggling. "Robert Carlyle or Sean Bean have nothing to fear from me. I feel I will never be a mainstream actor," he says wryly.
Bradley's performance was so natural it is hard to believe he was playing anyone other than himself. Thirty years on he's still "as thin as a beanpole" and although his hair is greying and his face lined, he exudes the same sense of barely concealed vulnerability that made you want to mother Billy Casper. The thick Barnsley accent is gone and he speaks received pronunciation in a soft, thoughtful manner, although he claims "another thing I have in common with Billy is that I sometimes have to work to find the words I want to say because basically I'm a kid who left school without any qualifications. I'm self-taught and insecure in that way."
Bradley grew up on a working-class estate in Barnsley, the son of a seamstress and a coalminer. Like his father before him, he failed his 11-plus and so went to St Helen's, the local secondary modern school. And although inspired by his metalwork teacher, he performed in the school pantos ("I played Sinbad the Sailor. It was as popular as the town panto and got great reviews") he didn't consider acting as a career. Then a rumour went round that a film was to be made in Barnsley, adapted from a novel written by Barry Hines, a former teacher at Bradley's school, and that the director wanted to cast ordinary people.
Three days after an audition - which he still remembers for the slap- up meal of salmon paste sandwiches and orange juice - Bradley received the letter that changed his life. "Ken Loach wrote to me inviting me to play Billy Casper, and the most wonderful thing was that it was written in purple ink. I'd never seen purple ink in my life!"
Bradley describes the seven weeks of his school holidays spent making Kes as "like being at Blackpool Pleasure Beach every day. It was one rollercoaster ride to the next". He'd get up at five o'clock in the morning to complete his paper round before being picked up for a day's filming.
In the evening Barry Hines would teach him to train the three kestrels used in the film (called Freeman, Hardy and Willis). Then he'd go home to have supper and learn his lines. "There were union rules but we tended to bend them. I was fourth assistant to the sound guy so if they were doing a wild shot I got to hold the boom for them. The thing Ken engenders in people is that we were all like a big family."
Apart from the obviously corresponding class, age and regional accent, Bradley insists there were few similarities between himself and Billy. He enjoyed school and, although money was often stretched, his family life was happy. Unlike Billy and many of his contemporaries, Bradley was comfortable communicating with adults. And, of course, he loved football so, in the film's memorable football scene, dominated by Brian Glover's bullying games master, "I had to stop myself looking like Gordon Banks making a save. But what Billy and I did have in common is that we're both loners. I still enjoy having time and space to myself. And I think I appreciate nature in the same way."
Indeed, when he describes the training process he went through with the kestrels, his Northern accent creeps back. He becomes so animated that, for a moment, I feel I've been transported into the scene in the film where Billy opens up to his classmates and teacher for the first time and describes the same process with the same passion. "Although I didn't know anything about kestrels, I did marvel at them. They're such stunningly beautiful birds. Freeman would fly straight at me, very low, perhaps 8 to 9ft off the ground. Whereas Hardy used to fly high and then swoop down."
When the film was released, Bradley was thrust into the public eye. "I went to the premieres in Sheffield, London and the world premiere in Doncaster and it was a very busy time." Since then he's tried to pursue a full-time acting career but it's been difficult. "My life is based around insecurity," he says now. "You go from one job to the next and it's a continual struggle to find work." He's taken occasional manual jobs to make money but bosses tend to frown on him if he asks for time off for auditions. "And if you do take on, say, a service industry job, then leave it to do a job in the profession, when the contract ends and I try to sign on they want to know why I left the service job. Right now I'm finding it hard."
His current project is a play - the name of which he wishes to keep secret - which he starred in and directed in Johannesburg, and would very much like to stage in London. But he's not finding it easy. "I've written to around 100 corporate companies to try to get people to meet me and allow me to give a 10-minute presentation. I'm looking for people to put up about pounds 5,000, which to them is peanuts. I got back around 60 letters, which is a great response, all wishing me well, some of them saying they remembered me from Kes, but no one was prepared to meet and take a chance on just 10 minutes out of their lunch break. I don't know where else to go now."
As Kes is reissued and returns to cinema screens next week, perhaps audiences will be reminded of his talent. In the meantime, he is philosophical. "As John Lennon said, 'Life happens to you while you're making other plans'."
THE CHILD ACTORS WHO DIDN'T STAY THE COURSE
Heather Ripley (right) starred in Chitty Chitty Bang Bang, aged seven, but never made another film and is now an eco-warrior.
"Filming took place in London and I missed my father in Dundee desperately," she says. "I remember getting up at 5am, being bored and having no friends. After the premiere people presumed I'd be an attention-seeking spoilt brat, 'full of herself' as they say in Scotland. Teachers came out with put-downs like 'we all know about your artistic talent, dear, but you're not on a movie set now'. The media constantly harassed me. I still suffer from depression, stress, anxiety and paranoia. I am by nature and upbringing completely anti-prejudice, fascism and warfare. I got pounds 7,000 for Chitty which has led to my resentment of the way Disney capitalised on my talent and my objection to worldwide capitalism. I now campaign for nuclear disarmament and have a website at www.bannerheather.com."
Mark Lester played Oliver and various parts until he was 18. He's now an osteopath.
"I thoroughly enjoyed performing, it was excellent," he says. But in the past he's admitted he didn't understand what a big deal a film role was going to mean for him. "Somehow the impact of my stardom never seemed to hit me. It seemed perfectly natural for a boy of 10 to be treated like royalty." It was a shock when his celebrity image began to fade. "Suddenly at 18 it all stopped," he once told reporters. "I was given access to a lot of money (about pounds 70,000), bought a Ferrari, designer suits and a house in Belgravia. I partied every night. It was terrific fun at first, but gradually I couldn't keep it up." In two years he'd spent nearly everything and started looking for a new direction. "I worked in a bar and trained in karate and considered training as an osteopath. It meant my going back to college and taking two A-levels at 28. I was thrilled when I was accepted to the British College of Osteopathy - it gave me more satisfaction than any of my film roles."
Mark Savage was Gripper Stebson, the bully in Grange Hill. The character was dropped after spawning copycat gangs. At the last count, he was facing eviction from his flat.
In an interview he said: "I'd dreamed of being an actor at 10. Suddenly I was on Grange Hill earning all this money. I was ecstatic." But he was often attacked for Gripper's bully image. "I had knives pulled on me. I lost a lot of my time growing up and missed out. People of the same age don't accept you when you're a child star. I spent my savings looking after my mother who died of cancer. Her death devastated me and I was left with no job or money."