Hoverflies Anasimyia lineata (Diptera: Syrphidae) mating in a buttercup (Ranunculus sp). This photo was taken in the dam at the lower end of the large lake. I often see A. lineata here. According to Stubbs and Falk (British Hoverflies, 2nd Edition, 2002) the species "is characteristic of lushly vegetated pools of mesotrophic or eutrophic character where plants such as iris, bulrush, bur-reeds, sweet-grass and willowherbs are present". Well, there are definitely irises, bulrushes and willowherb growing on the dam (I will have to look up bur-reeds and sweet-grass because I don't know what they look like).
Photo taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2010-05-16.