Not quite the classic Acorn shape, this one has had the egg of a gall wasp laid inside it xreating a Knopper Gall.
Something about the egg, or the way it is laid I'm not sure, causes a chemical imbalance which produces all these ridges and makes it look a little sickly. The wasp grows inside over winter and pops out in spring.
Apparently, it doesn't harm the tree at all, thankfully.
The wasps life cycle requires the next egg to be laid in a different kind of oak, which isn't native to the UK, but they were brought over last century for ornamental gardens, and these wasps have been appearing since the 50s.
