Oak Currant Gall - Neuroterus quercusbaccarum

Symptoms: Develops on the catkins and leaves of both pedunculate and sessile oak trees. There seem to be two stages, with the galls in the photographs being one stage and the spangle galls being another.

Cause: Caused by a small gall wasp.

Control: There doesn't appear to be much stress on the tree, so can be left in place. Also there can be many galls on each tree, making it very difficult to keep under control.

Thanks to OllieBeak at WAB for the help in IDing this gall. I found an ID guide from the NHM, available here as a PDF.