I am new to the world of gardening and have moved to a house with an established victoria plum tree, which is lovely.

After a good crop of fruit (although the early ripe fruit had maggots), the tree is looking very sad now. The bright green leaves have bright yellow spots blooming on the top surface. Under the leaves are little raised brown dusty spots, all over. The leaves are drying up, curling and dropping off.

Can you help me with what this is and what I should do about it.
Many thanks.