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Probably Ormyrus nitidulus (= Ormyrus tubulosus). They are really quite common around here on oak trees with knopper galls. There are some photos of this species half way down this page at Alain Ramel's Les Insectes site: http://aramel.fre...r-32.shtml
Ormyrus pomaceus is also associated with oak galls but not with knopper galls.
There is a key to to the UK Ormyridae in Oak Galls in Britain by Robin Williams (a PDF seems to be available at the NHM web site but I am saving up to buy a hard copy).