No Gypsum Plaster Please!
Dampness in historic properties is often caused by inappropriate material choice, frequently plaster. Using modern (gypsum) plasters, renovating plasters etc results in the absorption of moisture, prevents its release/evaporation and can accentuate and increase issues of dampness penetration and rising dampness - yes, contrary to a number of deniers, rising dampness does exist, albeit less frequently than often thought! - back to the issue!
Removal of the modern plaster, allowing structure (timber, stone, brick or cob) to dry out and if necessarily, replacing it with a lime (air lime - putty - or if absolutely necessary, a weak hydraulic lime) is likely to work wonders.