Aha! Good question from the two Johns!
Fact is, we're not quite sure : there is C. bullatus blood in them somewhere and in the past Roy Lancaster described them as 'bullatus hybrids' - and that's been good enough for me!
edit: Should have said - there may well be C. salicifolis blood in there too.
They are all slightly different, being seed grown from an original unidentified hybrid tree (not our tree).
We have quite a number of them, they're all pretty big now and, (fingers crossed) all healthy and flowering and fruiting well.