Bob H writes
K wrote:

On the underneath of the leaves the sooty stuff is nearly brown which
does wipe off easily.
As I said initially, the plant suffered this exact thing a short while
ago and we had to drastically prune it to get rid of all the scale? and
whatever else it was on the leaves and shoots. Now it come back again.

So if I prune it right back again, how can I be sure the same thing
won't happen again? What can be done to stop it happening?

What can be done is to find out what is causing it and then treat the
cause. It does sound like scale insect, but there's no point in treating
it for scale insect unless it actually has scale insect, so the best
next step is to have another good look under the leaves as I described.
If you were looking for moving insects, you wouldn't have noticed brown
scales looking much like bits of bark.
--
Kay