My semi-professional opinion is that the damp could be caused by moisture from inside the house, condensing in the cold corner. Most damp patches are caused by this, rather than moisture penetration probs.
The thermal resistance of the house structure in the corner may be lower than the rest of the wall, therefore forming a "cold bridge" which attracts moisture from inside the room.
Increased ventilation might help.
I sit next to someone in the office who might be able to offer a more informed opinion but he's not here very often.