Andrew Spence
If you post pics so we can have a look. If the wall is damp you need to find the water source, take all the plaster off the wall, fit a damp proofing membrane this is basically thin bumpy plastic before replastering. Does the wall that the damp is on back on to the shower? If so the shower may be leaking or may not have been fitted with a waterproofing system allowing the water to get trough the wall. If you post the picture that will be a start to finding the problem.
