Are you sure you don't have another source of damp?

Yes. The water was trapped between internal (which had been plastered
over) and external rendering and it was simply hydrostatic pressure
that eventually forced it into the house. In any case, unlesss every
cat for 50 miles is climbing on my roof, nightly, to urinate on the
rendering (I think I'd have noticed (:- ) rain is the only possible
source - and we've had very little of that in this area in the last 2
or 3 years. But if you have a credible alternative, I'm listening. The
novelty of bare plaster in the living room is beginning to pall.

Tony.