I have a camelia in bud but the leaves have gone black and feel roughPlease tell me how to deal with this

Where are you in the UK? Did it get badly frosted over winter or suffer from lack of water in summer?

Or is it sooty mildew?

Good point C.

Spot on. Aphids drop honeydew onto the leaves which turn black and create sooty mould. Try spraying with a dilute washing up liquid or if the shrub is not too large wipe with soaked cotton wool pad. When the weather starts to warm up, spray with a systemic insecticide to kill resident aphids.

Thank you all who replied. I will try wiping them then spraying in spring