Geoff wrote:
Our soil tends to be alkaline and we have four camelias in one bed which
have only been there five years and have been OK up to this year. I will
not describe what awful stuff was buried beneath where they are planted, but
suffice to say the previous owners had three bloody big dogs which never
went walkies!!

One camelia did not flower this year although the other three put on a brave
show but now something nasty seems to be attacking all them.

Many of the leaves of one plant are covered with what looks like soot and on
some of its leaves the soot has brown circular patches about 4-6 mm in
diametre.

Can anybody diagnose the problem and perhaps give an effective remedy
please.

It's caused originally by blackfly. They start attacking around early
May and las well as curling the new leaves they exude a sticky residue
which in turn is fed on by by sooty mould fungus. As soon as you see
blackfly on the new shooting tips run a thumb and forefinger over then
and remove. On a big shrub spray with Permethrin.