Pocket plum gall on blackthorn fruit. Many infected fruits in the area, some much longer and elongated, banana-like.
Caution: Do NOT use iSpot to identify fungi to eat!
Some fungi are very poisonous so a mistaken ID could have serious consequences.
No interactions present.
host is probably P. cerasifera based on green, non-spiny twigs, long fruit stalks and oval fruit shape. Redfern et al record T. pruni from P. spinosa and P. domestica. P. domestica is supposed to be a hybrid between P. spinosa and P. cerasifera, so P. cerasifera as a host would not be so surprising.
Yes of course it's not sloes, I didn't look at the picture properly. That tree is in a line of blackthorn bushes which made me jump to the wrong conclusion - thanks!
Lat/Lng: 55.941657, -3.264121
OS grid ref: NT211727