I moved into a flat about 5 years ago. The one bedroom had a problem with mould about a year later, this was treated with a fungicide at the time, but it keeps coming back. It turns out there was a problem with the damp course on the exterior wall, and that's been dealt with.
The mould keeps recurring so I decided to strip off the wall paper, when I took it off the base of the exterior walls had a foil liner. The walls above this liner were treated with what looks like sealant primer paint.
When I purchased the flat there was no mention of damp problems, but this liner and primer do