Good Friday started badly at the “summer” opening of Brockwell Lido in south London. But then, I was wearing swimming trunks and a head-condom, and preparing to plunge into water chilled by the coldest March for half a century with a man they call the “Polar Bear”.

When managers at the outdoor pool decided months ago to open up before the normal April launch to capitalise on the early holiday weekend, they might have expected splashing families to bask in spring warmth. Instead they got the big freeze and me.

The water sparkled in sunshine, creating at least an illusion of warmth. But it was soon shattered by head-to-toe goosebumps and the lifeguard’s thermometer. At 8am, the water was 3.9C. The air was two degrees.

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An hour passed before anyone else turned up. Peretz “Polar Bear” Milstein, an Israeli-born joiner from nearby Dulwich, arrived wearing flip flops. “Crazy? No, it’s healthy!” he said, stripping to his trunks before his Arctic plunge. As I walked out of the changing rooms, my extremities recoiled and my feet started to go numb.

Milstein, 63, advised against jumping in, as The Independent’s photographer had suggested (“the shock will be too much,” he warned) and so I inched down the steps. Even this way the icy jolt made it feel as if the Olympic-sized pool had been hooked up to the national grid.

As Jack Kitchen, a young lifeguard, looked on (he wore a thermal vest, a T-shirt, a hoodie, a winter jacket, tracksuit bottoms, hat and gloves) I did the unthinkable and sank all the way in, determined to swim a length.

By this time, Katie Kingwell, 29, from Brixton, had joined us, and floated serenely past me as I tried desperately to fill my shrivelling lungs. “Keep breathing!” she said. Fans of cold-water swimming talk in raptures about the rush of endorphins and sense of wellbeing it can cause.

Kingwell and Milstein are members of a core group known as the “icicles” who swim during the lido’s limited winter schedule, when snow and ice frequently bind the pool. “This is quite warm,” Milstein told me.

It didn’t feel like it, and my body was seconds from seizing up as I hauled it out of the water. Staff later told me that only 20 swimmers arrived on opening day, only one of whom was not part of the winter crew. I’m not surprised and while I’ll admit to feeling more alive than I have for ages, it would take me four hours and as many hot cross buns before I stopped shivering.