Mystery fruiting tree

Mystery fruiting tree

Observation - Mystery fruiting tree. Description: This was observed by a friend (I have permission to post!) in the grounds of Cheltenham College, so it may not be a native plant. It has got me stumped. The fruit looked a bit damson-like to me at first, but the ends are not smooth. Has anyone got any ide

This was observed by a friend (I have permission to post!) in the grounds of Cheltenham College, so it may not be a native plant. It has got me stumped. The fruit looked a bit damson-like to me at first, but the ends are not smooth. Has anyone got any ideas?