Getting out of bed on a cold February morning is hard enough but, as most of us are hitting the “snooze” button, some hardy individuals are already up and about, and flinging themselves into icy cold water. Winter swimmers are a special breed, who defy the weather and swim outdoors. For them the idea of a heated indoor pool, with its regimented lanes, chlorine, lockers’ stench and deafening acoustics, can never match the exhilaration of the open air.

Each outdoor water spot has its own flavour. At the Hampstead Heath Men’s Pond in North London, elderly intellectuals swap stories about Einstein’s love life by the still, green water; while the first wave of young swimmers to arrive on their mountain bikes at the Tooting Bec Lido in South London cut determined lengths through the leaf-littered gloom.

Sadly, many of Britain’s architecturally magnificent lidos have been shut (five in London alone over the past ten years or so), and the Hampstead Heath ponds are threatened with closure or entrance fees. The Greater-Mottled Winter Swimmer may be a tough species but it is also an endangered one.

While hunting down the specimens displayed here, I discovered that one female swimmer, who had been described to me as “attractive and about 36”, was in her fifties. Another, estimated to be in her thirties, was in her forties. It might be that swimming in cold water is the elixir of youth. Everyone I spoke to enthused about its benefits for health and state of mind.

And science backs them up. Dr Peter Clough, of Hull University, found recently that cold baths can help to reduce stress and increase mental resilience, while a study by the Thrombosis Research Institute in 1993 indicated that cold water bathing benefits the immune system, circulation and libido. What is universally agreed is that you have to acclimatise yourself to cold water; anyone with a weak heart would be advised to think before they leap. And don’t swim unaccompanied.

Initially it is hard to see how shivering in the murky Channel could make you feel smug, as Ruth (far right) puts it, but these swimmers are in on a secret exhilarating world. Roger Deakin, the author of Waterlog: A Swimmer’s Journey Through Britain, wrote of one dip: “In the night sea . . . I have seen bodies fiery with phosphorescent plankton striking through the neon waves like dragons.” Could anyone be so lyrical about a “thighs, tums ’n’ bums” class?

Gavin White, 36, radiologist at St George's Hospital in Tooting. He swims at the Tooting Bec, Lido, South London

“My father and I used to swim in Belfast Lough in winter, when I was about 7 years old. I didn’t particularly enjoy it, but I used to follow my dad to look brave. We moved to Dublin when I was ten and my passion for cold water swimming developed in the Forty Foot, an old gentleman’s seawater bathing hole.

“I have been swimming in Tooting Bec Lido for seven years and I go two or three times a week. My ambition is to swim in every sea on earth. Wherever I go, if there is water around I just have to throw myself in it.

“I have swum in Lake Tahoe, California, in the snow, in Lake Michigan in December and I’m taking part in the Finnish ice swimming championships this month, with a group from Tooting Bec.

“Swimming in cold water is almost spiritual. It sounds weird but time slows down and when you come out of the water everything around you is really bright and loud. You hear birds twittering miles away. Because the lido is surrounded by trees you have no idea that you are in the middle of a city.

“They installed hot showers recently and we protested as it’s wimpy. But we love them now.”

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