This was about 3/4 of the way along the hedge beside the River Calder at Cromwell Bottom, today. Each fruit has been affected along this shoot.
I did a search for "Gall on Blackthorn" and Wikipedia informed me that it's Taphrina pruni. It's sometimes known as Pocket Gall, as the empty, flattened, elongated fruit is pocket-like.(See white object at the bottom of the shoot.)
The same fungi infects other Prunus species, and a similar one infects Alder.