The soot is almost certainly a consequence of sap sucking insects so you need to deal with them first. You can go the full-tox route with proprietary brands of insecticide (but please check that the active ingredient isn't a neonicitinoid) or the green route using a spray solution of 100% soap (not detergent) and vegetable oil. If you can blast with a hosepipe first you'll wash away the soot. You may need to spray more than once.

Your Camellia is probably infested with Cushion Scale, common on these plants, and sooty mould is associated with it - google Cushion Scale and select the RHS page for how to treat it. You need to clear the scale infection or the sooty mould, whilst it flakes off on its own in July anyway, will return.

thankyou to urbanite and bamboo, it seems a lot of hard fiddly work but we will give it a try.