When I visit the Hampstead Heath Ladies pond, it is the height of the summer heat and there is a long snaking queue just to get in the water. The grass is filled up with sunbathers stacked almost one on top of the other, but the water itself still shows glassy black through the tiny dots that are the people swimming there. The queue is long but it moves fast, and it’s not long before we get our turn to clamber down the metal ladder and into the water’s darkness.The temperature is delicious, not cold as such, but a welcome relief from the sweat and chaos of the day. We kick away from the ladder’s base as the next in the queue descends. Despite the imposed order and all the hundreds of bodies, it feels calm. Most people aren’t here to cover distance but to cool off, to feel the water on their skin. Heads are up, looking around and chatting, not down in an earnest stroke. It feels eccentrically British, in the best way.

We swim the length of it and back before my friend gets out and I turn for another lap. I swim the circumference this time, jumping then laughing when the water lily stems brush my legs, when my foot connects with a mulchy bottom. It’s far too deep for accidental brushes just a few metres out, so it surprises me when my ankle connects. Ducks on the small jetty flap at me for getting too close; the shallows are their domain.The trees knit their leaves together overhead, pulling back in the centre to show a circle of sky. The water is teeming with chatting, laughing women. The trees crowd in at the edges, secluding it from other eyes. It’s private but joyful, secret but packed.

As I kick back to the ladder a duck, a grey fluffy crest adorning it’s head, makes it’s way in front of me. At least six people hold back to watch it pass, marvelling at its grace.

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