Damp Internal Wall on Second Floor

Hi I have very cold internal walls in our upstairs bedroom / under the attic stairs and on the external wall where the attic stairs are. In addition where there are picture frames on some of these walls mould appears on the back of the frames. The house is old with no cavity walls and while the downstairs rooms are fine its only upstairs we have the issue, the wall in question is the side external end terrace wall. Again, cant see any problems on the front or back walls. Any suggestions what the issue may be or a suggestion of what type of tradesman to ask? Im keen to avoid someone convincing me of things that might not help. A street away someones house which is in a similar position seemed to have their end wall coated / sprayed which presumably prevents moisture coming in through the brick work which im feeling might be a good idea here. thanks for anyones help.