Thanks both. No, there are no bugs inside, but there are stones, so it is neither an obvious infestation, not Plum Pocket (thankfully).
I am still bafffled by it, though. could it be weather related, do you think? Most of the fruit on the tree is now like it, and the old ones do seem to shrivel up in situ, but I do not know if that is normal because this is our first plum tree! Age of tree is unknown, but it is pretty established so over six years old I'd say. Each year it has had this on some fruit, but I think it is a bit worse this year. Still makes a decent jam, though, so it is not making them bitter or anything.
