Annobel I have just the same problem with a flowering crab apple - the fruiting one nearby is always healthy and currently weighed down with masses of fruit. The ornamental one shed most of its leaves last year too, and looked really sad, but I gave it another chance. When the leaves started dropping off again (having looked ok in early spring) I decided to spray with a fungicide (as I do with a rose afflicted with blackspot). Think we were a bit late this year, so the plan is to start the fungicide early next year as soon as the leaves appear and repeat as instructed on the packet. If that doesn't work, guess it might be for the chop, but I like to give plants lots of chances!
Would love to hear how you get on with yours, especially if you get advice from the tree surgeon.