If you have timber framing, there will be at least 50 mm of mineral wool between the external flue and the outer side of the timber frame. This is there for 2 reasons - to accommodate differential movement between the masonry and the timber frame, and as a fire barrier. This movement, over the 50-odd years, winter and summer, that the house has stood, has probably settled the vertical insulation leaving a small gap at the top where it originally met the insulation in the roof space, creating a cold bridge.
In turn this cold corner at ceiling level meets humid warm air and condensation occurs, creating damp in the plasterboard at upper level. It is a continuous cycle that occurs each autumn to spring, then dries during the summer.
Thus the 7-odd mild damp patches and the three-year history of poor paint adhesion in the same area of your daughter's bedroom, could possibly remedied by a simple rearrangement and possibly addition of insulation in the roof space.
It is so unlikely to be salts or a leak considering the internalization of the timber frame structure, there would be other more damaging indications of water ingress.
Another problem to look out for in the roof is condensation on the underside of the membrane, which may be dripping onto the surface of the roofspace insulation.
I'd get a roof insulation specialist, or two, to do you a free survey in case there are grants available to upgrade your 1960s insulation, assuming it hasn't been done already.