A male hoverfly, probably Dasysyrphus albostriatus (Diptera: Syrphidae). Presumably albostriatus because of the two white lines just behind the head. Note that the lines on the abdomen lean forward and are straight (not curved as those in Scaeva pyrastri and Eupeopdes luniger). The following photo shows the lines a bit more clearly:
And this is a female:
As with Eupeodes luniger, there seems to be quite a bit of variability in the thickness of the lines on the abdomen.
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2010-06-19 (the male) and 2010-07-11 (the female).