You don’t expect to find a lido in the centre of the city, ringed in by building blocks and rusty-bricked balconies. But it’s there, hidden in a seemingly normal leisure centre on a busy London street, rustling with city workers shooting up and down.

Because it’s central and because it’s almost a novelty, it’s often overcrowded. I’ve only visited once before, in the summer, and I ended up exiting before I’d done my planned number of lengths because the crowds were so plentiful I could barely swim.

That is one of the best things about lidos generally – they never seem as packed as indoor pools and people brave or silly enough to swim outside seem to have more intention of serious swimming than stopping and chatting and blocking the shallow end for those who want to turn there. It feels less stressful with less bodies to consider, less keeping out the way of those faster than you and less strategic planning to takeover someone slower. All this – the pool politics – can turn a supposedly relaxing swim into a stressful or frustrating experience. And that is one thing that seems to be reduced in lidos – less people, less politics, more community.

The Oasis lido is small, just three lanes take up the whole pool and the length is much less than my local 50m. And so, there is inevitably a bit of pool politics – the people who splash as if the water is an enemy to subdue, the people who just won’t let you pass, even when their feet continue to narrowly miss your head. But on a Monday evening in winter, the bodies are much reduced and it is still possible to plough up and down, with more enjoyment than annoyance.

The pool is the classic aqua pool-liner blue, but in the dark it glows with phosphorescence, the yellow beams of under water headlights shining vortexes through the shifting water. It’s never still with so many swimming costumed figures roving from one end to the other. Directly above it, the night sky is there, but on all sides the buildings rise – office blocks or flats, climbing up in the darkness like ladders of roughly hewn stone.

The grey paving slabs crackle beneath my bare feet, cold enough to split the nail polish. The water is warm, with half of it hanging suspended in the few feet above the surface, like low lying mist in the morning. Swim-hatted heads like life-buoys make steady splashy progress and I join the never ending circuit. As I swim, I can see the moon above to the left. It is just a sliver off full and the colour of age yellowed parchment, complete with all the cracks.

I swim up and down, the darkness increased by accidentally packing my tinted goggles, and the steady shot of swimmers, like the hands round a clock face, feels almost inspiring for once. I join in the race of reaching hands and kicking feet, narrowly avoiding those in front and below, until I’ve done as many lengths as planned.

I sit back against the slippery tiles and look at the lights shining through the water, the hemmed in bit of freedom in the centre of this urban jungle.

Please sponsor me at http://www.justgiving.com/swimbonnieswim