A Cynipid  gall wasp that causes common and familiar Knopper galls to form on the acorns of various oaks. 

[Note that parasitoids and inquilines may also emerge from knopper galls]

There are many similar species of gall wasp

We recommend that the gall-causer is verified by an expert before submitting to NatureSpot - take good photographs, note the host gall and emergence date, and retain a specimen

Knopper galls on oak

A recent introduction to the British Isles, first arriving in the 1960s and now found throughout England, Wales and as far north as Scotland.

Common in Leicestershire and Rutland.