Day 130/20. A most unusual spot.....

We went for a walk today around another small local pond, a mile or so from home. This one used to be a quarry, and is pretty big (well, its quarry size...). There were ducks, there were fish, and a few early but skittish dragonflies...... and then I spotted this.
Sitting on a fallen branch sunning itself. I have no idea if it's a Terrapin or a Turtle, but it's a decent size, perhaps 20cm nose to tail. A released unwanted pet? Is there a colony living in this pond? A most unusual discovery, I have never ever seen one of these in the wild in the UK....
Anyone identify it?