For years, I drove round the soul-destroying M25, exiting just after Hillingdon Lido, without knowing it was there. The car park and the somewhat bleak exterior don’t lead to high expectations. But then I step through an arch and – I see it. A long, flat expanse of water, hardly rippled by the solitary swimmer. A pluming fountain, white and rushing before me. And suddenly all I want is to be in that water.When I ask for a ticket to swim outside the staff raise their eyebrows and repeat that’s it’s not heated. I nod, that’s okay.

Getting into an outdoor pool always follows the same ritual for me, immersion in delicious bursts, feet, waist, shoulders, big breath and then the face. The water smacks the numbness from my cheekbones. The water feels so fresh I can’t tell if it’s chlorinated and almost taste it. The bottom of the pool feels a long way away. I strike out. The pool is soon all mine. It is coming up to closing time and not many people have chosen this cold water glory over the indoor, fully heated pool. I know they are missing out.

I stretch toward the far end, water still and glossy, dappling ever so slightly with a breath of air that passes. My insides are singing. With each stroke, my sides yawn, my arms pull me forward with the power and awareness that, for me, only comes in cold water. As I come up to the surface, I see the sky. Evening is coming on and the sky is colouring. Blue pales before deepening, melding through purple to peach. The water picks up the colour and, as I swim the ripples, dance in six shades of pink.

Swallows dip overhead. The birds chatter rises. They are claiming this day, this day that is almost over as I swim back and forth, marvelling at the colours of the liquid that holds me, stretching my fingers through the shapes of the ripples my body shakes across the surface. I get out, before either they throw me out or I start shivering. Numbed after a long day, now I can feel the edges of myself again. The water washes away all that has gone before it and makes the evening an adventure.

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