A female Thecocarcelia acutangulata (Diptera: Tachinidae). Identified using Chris Raper and Matt Smith's v5c, 2014 update of Belshaw's key that was distributed at their Tachinid Workshop in February of this year
Note the large Carcelia-sized eyes (which are hairless in this species) and the spike-like ovipositor. This species parasitizes larvae of the Essex skipper butterfly Thymelicus lineola. It was first detected in Britain in the mid 1980's and has now been found throughout the southeast of England.
Specimen taken in the field below Chazey Wood, near Caversham, UK, on 2014-08-31.