Hi paddywak,
This growth that has deformed the acorns is caused by an insect which is actually a tiny wasp, a species of gall wasp.
The galls it produces by laying it's eggs in the acorn are known as Knopper Galls.
These galls are becoming quite common and may affect birds like the jay that like to feed on acorns.
Hope you find that helpful. Tbird+
Thank you both for your rapid replies, I'd never really noticed them before and as there seemed to be a lot I didn't know if something was wrong with the tree