Pocket Plum Gall
Caused by the fungus Taphrina pruni, this disease infects plums, sloe, and damsons (as well as other closely-related Prunus), making some of the fruits grow in an oblong shape, and fail to develop a stone. Vascular tissues of the plant are also deformed by the fungal mycelium, forming witch’s brooms.
The local Mirabelles seem to be infected, although as you can see by the photo above, they still produce healthy fruits alongside the deformed ones. The fungus won’t kill the tree, but will certainly reduce the harvest.
The best solution is pruning out infected growth, and burning it to prevent the spread of more airborne spores.