This was a different visitor to one of the skimmers. A newt! I hadn't seen one since we had an old bath in our garden in Birmingham which was full of them.
I put it in a perspex baking tray with some rocks and rainwater and considered, for about forty seconds, making a pond. Then I remembered how difficult it is to dig a hole in our garden for a small plant, let alone a miniature wild-fauna lake.
Phoned Jonathan, local font of all wildlife knowledge, and decided, after a chat, to release it into a stagnant part of the small river Cornelia.
Trip out with boy, newt in jar, and directions for correct part of river. Released him/her, shed a few tears of joy at creature's enthusiastic swimming and headed back for tea and cake.