The black sooty deposit is from scale insects on the underside of the leaves above. You can wipe the sooty stuff off to make it look better and use an systemic insecticide on the insects or if your are an organic gardener your usual way of getting rid of bugs.

Why do you need to prune it. Is it winter damage or too large for its position.

Thank you for the soot remedy.
My other camelia had shed quite a few leaves and looked a bit bedragged and sad, though, may be a little pruning to give it some shape. Am I wrong?

Its okay to do a little pruning - Camellia respond better to little and often than they do to lots and rarely. Check for scale insect under the leaves and on the stems and branches - you can remove them by using methylated spirits and a cotton bud, dab each one to kill and lift it off, or buy some Provado spray treatment for scale. If the sooty mould is caused by scale, you need to get rid of the scale to get rid of the soot.

Thank you. I have got some methylated spirit and will give it a go, not today though as it is raining, before I get the Provado. I have had my Camellias for years and never had these problems before, must be the weather conditions these days.