Knopper Gall

... so I should really know something about trees. I went off round the park and found Oak tree with acorns - no problem here - but one tree had these huge growths

some googling later and its the Knopper Gall, created by, and home of, the Knopper Wasp. And like much of our wilfdlife a recent arrival, first found in Devon in the 1950's. One tree had quite a lot, and the next one none at all.