I'm not sure why, but when I was photographing this one I was convinced that it was a Robber Fly. When I looked it up afterwards, it's nothing like it. Instead, I think it is probably a Yellow Dung Fly, Scathophaga stercoraria. The larva of this fly literally eat shit, whilst the adults feast on other dung-loving insects. After finding that out, I felt quite dirty that I'd got so close to it.
*Title nicked from 'That's Amazing' on The Fast Show.