At one time we used to keep a "blip bank" a selection of small interesting items which could be blipped when all else failed. It is a long time since this slipped away. However tonight I spotted this gall which I had brought back from the canal to identify.
It is a Knopper Gall. Galls are caused usually by an insect which lays an egg on the tree. In this case it is a Cynipid wasp (ANdricus quercuscalicis). I was trying to remember if the Oak was Sessile or Pedunculate, however identifying the gall decided things for me. The wasp specialises in Pedunculate Oaks.
The wasp is a comparative new comer having arrived in the UK in the 1960s and since having spread throughout England and Wales having even reached Scotland.