Anyone who thinks that our factual TV ducks uncomfortable themes should watch Rain in my Heart this week. Paul Watson’s documentary about four alcoholics receiving treatment at a Gillingham hospital is utterly uncompromising, often physically repulsive and profoundly distressing.

Two of his subjects died during filming, one on camera. Their drinking has produced horrible physical symptoms, devastating mental problems and destroyed their closest relationships. It is extremely hard to watch at times yet the raw humanity of the film makes it compelling.

Paul Watson is a television legend, credited as the godfather of reality TV in Britain for his 1974 series The Family, which documented everyday working-class life.

For all his influence, Watson says he hates modern reality TV. “I made The Family to be socio-politically involved in a contemporary society. That may sound pompous, but reality television has lost its ability to reflect on society in anything other than a ‘Coo-er!’ way. It’s all manipulation.” And he tells an anecdote about an assistant producer holding up a card reading “Ask him if he wants a shag” to a woman on Wife Swap after a row with her swapped husband.

Later landmarks include The Fishing Party (1984) following wealthy supporters of Margaret Thatcher — “It was said to be her least favourite film,” he says proudly — Sylvania Waters, about nouveau-riche Australians, and The Dinner Party, which also ruffled right-wing feathers.

“People assume I’m an old leftie,” he says, “and I say, ‘No. I’m not. I wasted my entire life voting Liberal!’ ” Rain in my Heart is his first film since recovering from a near-fatal illness. He had a stroke while filming Desert Darlings (the closest he has come to a formatted reality show) combined with a serious prostate condition. “I approached about 80 hospitals about the film,” he says, “but they were all afraid I’d make a reality show or some kind of exposé.”

Access to the Medway Hospital, Gillingham, came “through the old- boy network”. He and a consultant, Gray Smith-Laing, both had sons at Tonbridge School. Smith-Laing approached the chief executive and gained permission to film on one ward. Several staff refused to appear on camera and only four of the alcoholic patients agreed to be filmed, hoping that others might avoid their fate.

Hesitant to participate at first, Smith-Laing uses the camera to get some home truths off his chest about the huge cost of treating alcoholics and the lack of psychological support after they are discharged. Watson refuses merely to stand back and observe. The original idea had been to make a film about the process of making documentaries, he says, and he often appears on camera, sometimes filming. But as he gets to know the alcoholics and their loved ones, he finds he cannot stand aside. He gets emotionally involved, and his questioning sometimes borders on therapy.

Watson says that reaction to the film’s premiere at the Sheffield International Documentary Festival this month varied. Young film-makers were shocked by the film’s toughness.

Watson responds: “They are making films per the orders from the suits, and the suits have never been among the realities they send them out to film. They go armed with a clichéd view of what they’re going out to see, and unless they bring that clichéd view back, it’s not acceptable.”

He discovered film-making at the Royal College of Art and has pursued it to the detriment of his private life. (He has talked openly about his marital difficulties and unhappy relations with his older children.) There has been a burning desire to do something meaningful. “It’s about making markers for history. I just hope that in 50 years, someone’s going to look at my work and say ‘Christ! He had his finger on the pulse!’ ” His old friend Peter Bazalgette, of Endemol, makers of Big Brother, “is cynical beyond belief”, says Watson. “I used to play cricket with him. Win is the only objective, and it doesn’t matter if you bowl underarm or put chewing gum on one side of the ball — you win!” So how does it feel to be called the originator of Big Brother-style reality TV? There’s a pause. “Why would anybody want to be known as the godfather of such bastards?”

• Rain in My Heart, Tues, BBC Two, 9pm

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