Thank you for your question, Pam. This is at least the 5th question that just I have seen about the same thing in the same plant, all in the U.K. I cannot see the plant tissue clearly, so cannot tell if there are spots, or fungal growth. Here are diseases that we see here in the Pacific Northwest: https://pnwhandbooks.org/plantdisease/host-and-disease-descriptions?title=Photinia It is also one of many plants susceptible to verticillium wilt: https://extension.umd.edu/hgic/topics/verticillium-wilt I suggest that you find a local plant clinic that has the resources to examine and test both the plant and the soil it is in, for pathogens. The Royal Horticultural Society might be of help. Good luck!