We had damp walls in our house when we bought it (late Victorian terrace). It was on the internal party walls. I am pretty sure it was caused by a combination of…
No dpc
No ventilation through floor (modern laminate placed onto floorboards) and floor very close to the ground beneath
Previous owner insisting on having thick lining paper everywhere
At the time our mortgage company required us to have it sorted within 6 months with guarantees etc
What we ended up doing was…
Ripping off render and plaster up to waist height (which we did ourselves)
Paying a damp proof ‘specialist’ to squirt some stuff into the walls. Cost about £500 for an hour’s work, including the certificate.
Recovering with a lime render and plaster, no lining paper!
Binning the cheap laminate and going for stripped floorboards to get a bit more ventilation.
I actually found the same dpc cream available cheaply online which I have used since elsewhere. ventilation and letting the walls breathe are the main thing though I think.
http://www.permagard.co.uk/aida-dpc-injection-cream-310ml-cartridge.html
5 years later, no more damp
This will be my likely course of action!