Check beneath the leaves for cushion scale, see link below. Also for a heavy infestation of aphids, both of these create honeydew, and sooty mould often occurs on built up honeydew. Treat the plant for whichever infestation its got, and you can either wait for the sooty mould to disappear on its own,which it will over time, or gently wash it off with horticultural or mild soap and water solution

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They do look like a sooty black mould, Thank you for advice. Regards Ann.