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.

I was looking for Dodder, a parisitic plant, which has just been reported by Portia Fincham, who found the Yellow Birdsnest at Todmorden. It was 6 years ago, she admits, and it was on this hedge near the furthest kissing-gate into Tag Loop. (Nearest the railway.) She did note it in July, so it was unlikely to be visible today, and it wasn’t.