A male scorpion fly, Panorpa sp. (order Mecoptera). The females do not have the large red 'sting', just have a red pointed tip at the end of their abdomen (see here). There are 3 UK species of Panorpa but these are difficult to distinguish without the aid of a microscope.
In the past, I have found scorpion flies very difficult to photograph: they seem to like hiding in among twigs and leaves, and when you go anywhere near them with a camera they immediately fly off. Today the weather was a bit cool and maybe that is why this fly was a bit slower than usual.
Photos taken in Reading University grounds, Reading, UK, on 2009-05-17.