The 2cm holes were drilled into the walls - one at the top of the wall, one just above the skirting. The cavity wall was bridged with a piece of plastic pipe [as used in wc overflows].
Some plaster was removed around the hole, a small piece of nylon mesh was put in over the hole, then replastered around the hole to fix the mesh in place. Then it just needed touching up/painting in.
From the outside the holes could not be detected, even inside noone would notice them until they searched for them.
It worked for us - after using dehumidifiers and everything else we could think of without real success.
Comments
Thanks Quatro - I might give this a go
MilnrowLad