In the nicest possible way, this gentle and forthright film is a kind of home movie, almost a series of hyperlocal video-diary entries, full of uncomplicated happiness in a way that most documentaries aren’t. Mixing interviews, ambient material and wonderful drone footage, this is a tribute to the bathing ponds of Hampstead Heath in north London – the women’s one, the mixed one and the men’s one, and to the people of all ages, identities and backgrounds who use them all year-round. They are people who have developed what might be called a virtuous addiction, a predilection for the cold-water endorphin rush, especially in the winter when the heath is at its most beautiful and the water at its coldest.

This is Britain’s equivalent to Germany’s freikörperkultur. I confess that I have sometimes ventured into the men’s ponds with my son, who satirically says that the open-air changing area here is the nearest law-abiding Londoners will get to the prison yard – joking, of course. These are democratic places where there is a great levelling. Nobody is wussy or evasive enough to use those bodysuits modelled by the former prime minister David Cameron, and interestingly, the swimming culture is a liberation from the pornification of the human body: it is a place where unselfconsciousness rules. Many of the swimmers interviewed in the film talk rather movingly about their own emotional crises, their encounters with illness and bereavement, and the role swimming has played in dealing with it all. One interviewee talks about how the Russian term “living water” describes the experience of swimming there. I agree.