what I have done on a couple of old terraced houses with issues like this was to batten the damp wall (with DPC behind the battens to keep them from getting damp) then plasterboarding onto the battens, creating a cavity. Makes the room a couple of inches smaller though. It's a job easily done by anyone who can swing a hammer, just leaving a bit of plaster skimming, which may cost a couple of hundred, or £1500 if you live in London.
A lot of younger people and kids spend much of their time in their rooms, breathing, with doors and windows closed. But the moisture is less likely to condense on the above mentioned wall.
A small dehumidifier running when room is unoccupied would help too.
Obviously other potential culprits like roofing, guttering and pointing need to be checked out too.
I just dropped in, to see what condition my condition was in
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