If its got sooty mould, its probably got a cushion scale infestation, and these inhabit the underside of the leaves, producing sooty mould eventually on the upper surfaces. Treat the cushion scale and the sooty mould disappears, but unfortunately, the best time to treat is around June/July, see link below

https://www.rhs.org.uk/advice/profile?PID=483

Thanks Bamboo, I'll take a look. Meanwhile I shall try washing it gently as its causing the pic window frame to go black.

Check to see if you have an army of ants marching up and down the stem of your camelia plant. The ants actually carry the aphids, scale or mealy bugs to your plant to farm the "honeydew" they excrete by sucking the plant's juices.

The black sooty mold you see is fungus that grows on the honeydew excreted by the mealybugs that are carried by the ants. Insecticidal soap will kill the scale or mealy bugs, but you have to stop the ants from coming to your plant in the first place.

A number of products at your locale garden center will do the trick - tangle foot is one or encircle your plant with a couple rings of diatomaceous earth if you can get some.

Ants are not an issue, if there are ants, they're there simply to harvest the honeydew, but you're unlikely to see any ant activity at this time of year anyway. Treat the scale and the ants disappear.

No they don't Bamboo. Ants actually bring the bugs to the plant. You don't give much background information Cammomile, but ants are a primary cause. You'll have to check as I mentioned above.

Can you provide a reputable link to this information please? I am unable to find any reference to this on the web...

Sure
http://www.timberpress.com/blog/2012/05/8046/

Interesting - that's not a primary cause of scale infestation here, for sure, but then you have a range of ants over there we don't have here.. Ants on plants here are merely a useful indicator or symptom of an aphid or scale infestation that already exists.

It could be scale, aphids or mealy bugs. Ant husbandry has been going on for over 50 million years. I'm surprised this is news to you. Here is a video...

https://www.youtube.com/watch?v=DnpJibC5iA0

Won't be mealybugs in this country - apart from Phormium mealybug in mild areas, they're only a problem on houseplants.

I couldn't tell if said camillia was in a greenhouse or planted in the garden.

It's in a trough facing east, sheltered between our garage and the one next door. There are no ants but I have seen what could be the odd mealy bug on the underside of the leaf above the sooty stuff. I shall go out and try washing the leaves when I get five minutes and treat the problem next year with a bug spray. Thanks all for your input and a Happy Christmas to you.