Saw quite a few of these striking flies around reedy ponds in East Sussex on 20th Aug 2014. Pretty sure I also have a specimen of the same from the pools at Moccas Park but haven't even worked out what family it belongs to yet. Admittedly I haven't tried but can someone spare me the effort by recognising it straight off? Thanks!

Edit: Dave Gibbs tells me it's an easy one - Sepedon sphegea (Sciomyzidae).