Had me completely baffled for a while but looked a bit like the more familiar lacewing larvae, and when I googled "Coniopterygidae larva" images, there are several similar to this. From oak red-rot in Herefordshire. Dreadful photo admittedly but it is constantly on the move. They may be common as muck but I've never seen (or never noticed) one before, and they're pretty striking insects!
Post script: reared through to adult on 22nd July 2014 - a male Coniopteryx tineiformis.