Sorry Chris, Posted last effort before reading yours. Have read some
of the suggested stuff before and my guess would have been option B,
because there is no damp in any direction around the problem areas,
the DPC appears to have been done properly, internal wall, problem is
all over where dabs are rather than just bottom couple of feet, and
salt crystals are clearly to be seen on the damp patches. The chimney
was very sooty and damp when opened up and probably not given long
enough to dry out so i'm thinking dabs have leeched salts into board
which now suck in moisture on wet days. Does that sound plausable?
If the cure is the same i guess the cause doesn't matter so much - if
proposed replastering will sort the problem either way? Cheers