A tachinid fly of the species Tachina fera on a water mint flower (Mentha aquatica). Gonia divisa, another tachinid fly, is very similar, but has black legs instead of red ones. Tachina fera larvae parasitise various caterpillars. Photo taken in Reading University grounds, Reading, UK, on 2008-08-23.
A couple of years ago I took the following photo of another tachinid fly, probably of species Eriothrix rufomaculata.