Tooting Bec Lido is one of the prides of London, at least as far as the pools are concerned. At 100 yards long, 33 yards wide, it is the biggest open air pool in the UK. The Pantone primary-coloured doors of the wooden changing huts are iconic as they stretch out like a line of film stock above the blue water.

My favourite thing about the lido is it’s surroundings. On the edge of Tooting common, it’s surrounded by tall leafy trees. It doesn’t feel like it is in the city at all. It is unheated and a few people are wearing wetsuits. Odd leaves float crumbled at the edge of the water, where a lifeguard flicks then out with a long pole and a net. The turquoise pool liner that gives the water it’s summer colour has no lines to interrupt the bottom. There’s only one lane, one lane rope, and apart from them the wide expanse of water is un-regulated. The lack of these pool guides and the trees overhead heighten the effect of swimming under the sky, the unavoidable pool edges quieter when the rest of the man-made impositions are gone.

The water is quiet, the sky is bright but grey. Trains thunder down one edge of the pool, seen only through the fence where the changing huts stop. A mural at one end is hidden by construction work, but the fountain burbles and the picnic tables are half-full despite it being a cloudy, fairly cold Tuesday morning.

I swim up and down, feeling far away from the other swimmers, looking down at specks of sand shifting along the bottom. It feels like summer when I look down, though there’s no sun to paint rainbow wavelets on the bottom or to block out my body in shadow beneath me. Up above, it feels like British springtime, fresh and holding its breath for those few days of sunshine. One of the wonderful things about being in the water outdoors is being able to feel the seasons shift and ripple around you.

I’m raising money for the Alzheimer’s Society. Please sponsor me at http://www.justgiving.com/swimbonnieswim