please see my thread on damp issues in Victorian dining room.
In summary, from first hand experience of fixing a major damp issue:
Penetrating damp is a big cause, root causes need to fixed. e.g. gutter, floor too high outside, etc.
Rising damp
100% is real, and if you don't have a DPC,
will make your walls damp. If your DPC is bridged, i.e. render or flooring is connecting the walls above and below the DPC, the damp will spread upwards. (I don't understand why rising damp is even a debate)
@paulrbarnard can you please post plenty of pictures, inside and outside, of the wall in question? and perhaps a wider view of the outside so we can see what's around the wall?