I seem to have misplaced my big book of insects, and the various AI-based ID sites I might look at just sort of shrug. It was walking around on frogbit leaves in the meadow ditch that hosts the best of the dragonflies and the fen raft spiders. When disturbed it flew weakly and not very far, before going back to walking around. Unfortunately I couldn't persuade my camera to focus on it properly, but it looks like it might be more distinctive than most of these things? It was seen yesterday.