I have damp on internal walls in-line with roof joists

sounds like you have a leak in your roof, depending on what tiles are on your roof as its the original roof its well past its sell by date, originally the battens would have been torched with lime mortar to catch any water getting under the tiles but this will have long gone allowing any water to track down the trusses into the wall plate & wall, really think you need to be looking at felting underneath your tiles to prevent further damage the roof is ninety year old its well past retirement. most insurance companies recommend you replace your under felt every 25yrs to protect the woodwork the fascia's are usually the first sign the roof is leaking rotting the boards from behind as water runs downhill and stops at the fascia to seep into the brickwork. putting vents in the fascia's is never a good idea its generally over fascia vents or soffit vents. good luck Alex

Remove all the roof tiles and have new felt and Barton installed. Whilst doing the work have Eve trays fitted as well. Then put all the roof tiles back. That would sort the problem.

It could quite easily be condensation. Pictures would be required to ascertain this.

Who said the room was original and 90 years old