We break our journey here, hoping to turn a day of driving into more than just hours sat in a hot car in traffic. We aren’t sure we will find it at first, but as we get close it’s clear this is a well known spot. People with dogs, families, groups of teenagers, couples – the grass is dotted with sun-basking, chattering, music-playing groups long before we see the river.

The “lock” itself is a pool, deep enough to swim but shallow enough to stand, with a rush of water falling into it from a higher level at one end. The water swooshes and foams and children ride down the drop on inflatables, screaming with excitement and fun-filled fear. The water just after this pool looks shallow. I want more of a swim, so we walk upstream a way.

I get in at a point where the bank slopes easily. The water comes to just above my knees, the current comes fast, and I squat down, let my legs float up, and let myself float downstream, buoyed by the current. Liam walks beside me on the footpath that runs along the bank carrying our stuff.

The river is thick with silky green weed and, as the water level is low, I try not to kick too much and disturb it. It brushes along my stomach and thighs anyway, slightly slimy, slightly tickling. The banks rise up high either side of me, flecked with wildflowers and water weeds. A cow stares down from a field, thoughtfully chewing as I pass. The water pushes so fast I hardly have to work at all, and before I know it, I’m at the lock.

I stand up for a moment to asses it, to make sure there’s no one behind me and no one I’m going to crash into if I go through. And then, slightly nervous, I push off and out and down through the tumble of white water, diving under the surface, kicking hard and coming up again in the circular pool that is the centre of the action. I go with the waters push and follow the river around the corner. I’d like to keep swimming, but the water is too shallow and soon it’s rocks grazing my knees rather than weeds, so I pull myself awkwardly to my feet and wade back to the bank to dry in the sunshine.

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