Sciomyzid Fly
A Sciomyzid fly, probably Limnia unguicornis (Diptera: Sciomyzidae). The only other Limnia sp found in the UK is Limnia paludicola but that has uniform brown longitudinal band ('vitta') on the top of the thorax. Furthermore, it has been suggested that L. paludicola may not be a truly independent species and could eventually be found to be just a variant of L. unguicornis.
Photo taken in the field below Chazey Wood, near Caversham, UK, on 2011-05-16.