OK Boogie, me again.
Well, you should have a pretty good idea what's going on now, thanks to Karl's excellent analysis.
The bottom line is that the damp spots are most likely rain or condensation...... probably a bit of both in my experience. I guess Karl would go along with that too.
Houses of solid, dense masonry are the worst kind to deal with, I'm afraid. If I'm renovating one, I invariably treat all exterior walls by fitting an insulated timber frame wall inside each outside wall, with 50mm ventilated cavity. There are plenty of other ways of course. Easy when the house in empty, but disruptive for you. Simple DIY job though!!
External insulation would be wonderful ..... in short, timber fixed to wall, insulation between timbers, wire and render to finish......... then paint ...... like a new house!
(Bit more tech than that, but that's the basics.)
Also, the old masonry acts like the biggest storage heater in the world 'cos it's inside the insulation. Warm house, no damp, lower bills.
Alright, so much for the wishlist!
In the real world, all I can suggest is something like this ...........
http://www.decorating...k/viewprod/b/BITSUWS/
DIY application, lasts for several years before needing re-applying, works very well.
I've had to use it in situations like a porous chimney stack where rain drips into the fireplace, and the client is too tight to have the stack rebuilt. Works a treat though.