For me, the first fifty yards were like being in a fight, or having a tooth pulled without anaesthetic, or any experience where your consciousness contracts to the immediate struggle to keep breathing. Your skin feels as if it’s being stripped off you with wires and your head aches. There is no space to think of anything. Abandoned by your higher mental function, you regress to the state of a floundering reptile. The first few times you do this, it seems like pure insanity. Gradually, experience shows you that this moment will pass. As you relax, you begin to notice that there is no pain in the place where the breath enters your body. If you can abide there long enough, by the time you reach the midway point of the pool the pain has not only diminished but been succeeded by a sensation of well-being that verges on euphoria. On the return length, you are no longer cold. There is no feeling whatsoever in your lower body. Hauling yourself out of the water, you experience the air as warm. You can then walk the fifty or so yards to the sauna – a gift from the Finns on the occasion of the lido hosting the World Winter Swimming Championships in 2008.

At this point, you know that nothing you have to face that day will be more difficult than what you have just done, and whatever anxiety or stress you’ve been carrying has been purged from your system.

The sauna comfortably seats eight. At the times of day when I swam, there were usually no more than three or four other people in it. Many of the long-time winter swimmers regarded it as an unwelcome novelty and talked fondly of the old days when the amenities of the club offered nothing more than a cold shower and a cup of tea.

I should point out that there is no evidence that swimming in icy water is good for you. Only cod science and the attractive but unproven intuition of hormesis – the notion that something bad for you in large doses is good for you in small ones – suggest otherwise. In my first winter, I contracted a stubborn case of bronchitis for which the doctor prescribed steroids and antibiotics. I felt sure the swimming was to blame. And yet, all the other swimmers seemed in rude good health. I, in my forties, was one of the youngest. Some looked decades younger than their calendar ages. These people seemed to have found in the freezing water the elixir of eternal youth.