Terellia tussilaginis (family Tephritidae, order Diptera), a gall fly, on burdock (Arctium sp.). This fly was so persistent in hanging around this burdock flower head that I thought it might be a specialist parasite of that plant, and it turns out it is (as indicated on this page at Bioimages UK). At the time, I suspected that it might be a female laying eggs on the flower head, but when I got back home I discovered that the females have a large red ovipositor, so it must have been a male. Maybe it was guarding the burdock plant so as to mate with any females that visited it?
Photos taken in Reading University grounds, Reading, UK, on 2009-07-16.