In reply to Philip:
In time honoured tradition, I'm going to suggest something different to the answer you sought.
Try a quince; a fruiting quince, obviously, but it pays to know that there are fruiting and flowering quince and though the flowering quince do fruit, they're much harder work than those from a quince bred for fruit. You can make some lovely marmalade with quince and orange, as well as the other stuff for which quince is better known.
Otherwise, for plum trees I'm all for Victoria plums; ours is no trouble and fruits well. It may not be unusual but they are tasty,
T.
T.