Help needed - Diseased Morello cherry tree

My morello cherry tree is 4 years old and is situated on an east facing lawn which gets some sun but often damp from the Welsh climate. There was plenty of white blossom as usual about 6 weeks ago. Now the blossom has fallen, the tree suddenly looks very unhealthy. The fruit has either not developed or already falling off and some of the leaves are yellowing with black / brown spots. (The tree produced some fruit in its second and third years, and I cleared the leaf fall away from the ground last autumn.) I'm not sure what it is or how to treat it? Should I prune the tree now to remove the damaged leaves or is there another treatment available? Any advice would be gratefully received, thanks!