Hi Goosey, as you mentioned it was on a twig it is probably an Oak Marble Gall.
According to
Galls Page 1
"Each gall normally houses one larva of the gall wasp, Andricus kollari - and possibly, several inquiline (foreign squatter species) intruders. When mature they vacate the hard woody sphere through tiny holes similar to 'woodworm' holes."
I saw some red devils horns looking galls on a sycamore leaf on the way into work this morning. They were the other side of some railings in the Liverpool Cathedral Gardens, I was running a bit late so I'll have to investigate at the weekend.