The wall itself is around 100 years old, and was ~2 years ago covered by 10-20mm plaster and then two layers of plasterboard, with the first plasterboard foil backed (no idea why):
|------------old brick--------|-plaster-|#-p. board-|-p.board-|
The '#' denoted the foil backing.
Oddly enough we have started getting dampness on the wall, with a dampness meter (when poked into the plasterboard) reading >90%. But more oddly the old brick wall behind the plasterboards is virtually dry! If this was rising damp from the ground shouldn't the brick be wet and the outside plaster/plasterboard progressively get drier??
One dampness expert says "we believe that it is contaminated wall plaster in these areas" (no further explanation) and recommends a £600 treatment for less than 2 linear metres of wall...
Another explanation was that the plaster is drying out and expelling water, and due to the foil backing the moisture has nowhere to go except to the edge and then out/up. But shouldn't the brick wall then be equally wet as the plaster?
I first suspected my plumbing but that seems to be all in order...