Seen on on our kitchen window sill a few days ago, a fever fly, Dilophus febrilis (Diptera: Bibionidae). This species is distinguished from the similar Saint Mark's fly (Bibio marci) by the ring of downward-pointing spines on the lower ends of the tarsi on the fore-legs (visible in the above photo). As is normal in Bibionid flies, the male has much larger eyes than the female (see here), so much so that you might mistake them for different species. This specimen is a female.
Photos taken in Reading, UK, on 2009-08-31.
It turns out I had actually photographed a male just a few days before without realizing they were the same species. See here.