I bought a couple of plants yesterday which had black spot on the leaves. The salesman said they just needed planting out straight away as they were stressed, and the new growth would soon replace the diseased old growth. Is this likely? If so I will get some more as they were greatly reduced...
The black spots are most likely "entomosporium leaf spot", a fungal disease common with Photinia, India hawthorne, evergreen pear and some other plants.
The nurseryman's advise may or may not prove accurate, as this disease can spread by rain and wind blown spores from the existing infections on older leaves, to the new leaves in spring.
The old-school recommendation is to spray with a chemical fungicide, of which I find the systemic fungicide 'Banner Maxx' (or a generic brand with the active ingredient 'propiconazole') to be effective, depending on stage of growth, climate and timing of the applications.
The newer generation of organic-program-compatible materials should be considered. I have effectively used a biological fungicide for leaf spot, but it is preventive and not so much a cure, so you must start early as new leaf buds form and start to break open. I use Double Nickel Biofungicide/Bactericide from Certis USA, or a Monterey brand equivalent like this:
You will have to search for what is available in the UK.
Hope that helps, Don