Had the same problem with the wallpaper stains, you'll see by my last topic thats it's back to the plaster time. Start with one layer of polystyrene veneer all over the external facing wall and see if this has any effect with the condensation on the window. I laid two layers and have reduced the condi' problem by 99%. B&Q knock the gear out approx Â£12 gives you 10 sq m.
Good idea to deal with the condensation first. Addressing the effect is a continual job whereas addressing the cause need only be done once. Take a look at the condensation project on this website and introduce some ventilation to the area