In reply to gethin_allen:
I see you've answered a few of my questions already. I suggest you hold off of retro-fitting a dpc for now and do a bit more research. There are situations where they are appropriate but my research indicates that it's not very often.
Unforunately I'm renovating my house and don't have much time to commit to explaining everything but I suggest you do some research on traditional building techniques. These houses were constructed with breathable materials (such as lime mortar) which will absorb moisture but also allow it to evaporate. The building techniques were not perfect, windows weren't perfectly sealed, there were gaps in floorboards, etc, there was ventilation there.
The use of concrete and modern plasters on these buildings are creating water tight seals that aren't allowing moisture to evaporate. Your render is most likely concrete which is initially water proof but is very liable to cracking and once the moisture gets in behind it, it won't let the moisture escape. Similar situation for modern plasters. If they are walls that get very cold, you can get issues where the dew point can occur within the wall itself (the temperature at which vapour will condense).
This is a good website and can get you started but there's lots of good information out there;
https://www.homebuilding.co.uk/?s=damp
Good luck.