A small grey Tachinid fly on hogweed. Probably a Siphona sp, possibly Siphona geniculata (Diptera: Tachinidae). Siphona are so-named because of the thin proboscis, or siphon, which they suck nectar up with. In the above image the fly has its (bent) siphon inserted in the hogweed flower.
This image shows the abdomen has an orange patch on each side. Such orange patches are also shown in images of Siphona geniculata near the foot of this page at Alain Ramel's Les Insectes site. This and the fact that the above images were taken near the end of September tend to suggest that this fly was S. geniculata. Most of the other British species fly only in the spring and summer, and S.geniculata is by far the commonest British species anyway (see here). However, Siphona species are reputedly very difficult to identify properly, so this is only a tentative identification.
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2010-09-30.