Even people who don’t normally have issues with damp are having probs. this year, that looks to be penetrating damp, so would need to see pics of the outside.
Have you had the chimney issues fixed ?
that would be the first course of action..remember that properties of that age (20’s) were designed to breathe, so waterproofing “Creams” are likely to exacerbate the problems.
You may find a de-humidifier beneficial..
Bedrooms particularly seem to be prone to damp, as you will often spend 8 hours a night in there breathing out damp air.
Regardless before shelling out 15K do as much research as possible.