Pristiphora monogyniae is a sawfly whose larva causes a downward leaf roll gall on the leaves of Blackthorn.  The leaf margins are only slightly thickened, and are yellowed.  Often the leaf has a sickle-shaped bend.

Several species of aphid causes leaf distortion on blackthorn and plum/damson shoots, but are not galls. 

Gall  Adult

The leaf roll should be opened to check the gall-causer and eliminate the possibility it has been caused by an aphid.

Wherever Blackthorn is plentiful.

Galls occur in summer and autumn whilst Blackthorn is in leaf.

Widespread, but not well recorded in England.

Status in Leicestershire and Rutland not known.

https://www.sawflies.org.uk/pristiphora-monogyniae/