The attractively marked hoverfly, Myathropa florea, sunning itself by the pond, Oct 12th '09

This is a male, because its eyes meet on top of its head. It is a harmless hoverfly, but its markings mimic those of a wasp. This probably gives it some immunity from attack by bird predators which confuse it with the stinging insect. It is a particularly common hoverfly of wooded areas as it lays its eggs amongst decaying leaves in rot holes in trees. There are doubtless such suitable sites provided by some of the park trees.