Confidence: It's likely to be this, but I can't be certain.
Notes: Chinery (2007)* p.228-9 gives two possible candidates:
(i) Torymus nitens (Torymidae) which parasitizes the larvae of oak gall wasps and (ii) Pteromalus puparum (Pteromalidae) which is parasitic on the pupae of white butterflies. White butterflies are certainly around on our site; oak galls less certainly so (to be checked).
* Domino Guide to the Insects of Britain and Western Europe