When we first built the pond in 2005 one of the early visitors was a water measurer (Hydrometra stagnorum). I assumed it came in on a plant but could easily have flown to the pond. Over the last two years I’ve only seen one or two a year. They run across the surface film and feed on mosquito larvae which they spear from above with their mouthparts.
However, over the Bank Holiday weekend I saw four in two days. They’re pretty difficult to get a good photo of as they don’t tend to stand still for long. This is the best I could manage: