@Drywsdad and @Abby2, please keep in mind that evergreen shrubs and trees drop their older leaves in Spring, when new leaves are forming. However, I think the main problem is formed by the clay soil which your tree/shrub is planted in. It creates the wet conditions that Photinia's hate and is the main reason for leaf spot to occur, You can read more about that here: http://www.horticare.net/PDF%20Files/Chemical/Photinia%20Leaf%20Spot.pdf
Photinia's want a free draining, slighly acid soil, which heavy clay isn't. And I'm afraid that re-planting the tree with ericacious compost in the same planting hole will not resolve the problems you're having because the alkaline clay soil will mix with the compost after rainfall. Besides that, on a damp, heavy soil a planting hole backfilled with coarser material, including a lighter natural soil or topsoil mix may act as a sump.
My advice would be to either create a raised border completely filled with ericacious compost, mixed with a sandy topsoil or plant Photinia's in a big tub with holes for drainage. If that's not possible you'd be better off planting something else, for instance a Portugese Laurel.
Sorry for the long post, but I hope this will help you decide what to do.