Kate,
If it's an outside wall as you say the dampness may be from a leaking gutter - wait till it rains (not a long wait) and have a look. Is the ground level higher than the damp course? More probably though...
The wall in shadow will be cold and the air inside warm and wet. Condensation will form on the inside surface of the cold wall especially if it's a kitchen, bathroom or bedroom - steam and hot breath. Keep bathroom and kitchen doors closed and the rooms well ventilated - open the widows for an airing when you can.
If you can warm the wall up - even a bit - it will help.
If the damp is from outside then the landlord should stop it at source. If the damp is on the inside, from condensation, then if you put a proprietary product onto your cold wall the condensation will still form on top of the cold sealer!
Search for Jeff Howell - he writes extensively on building maintenance and is excellent with dampness.
Richard Buxton
October 2008