Before you involve contractors to solve a damp problem, have a good look at your walls. It is only in recent decades that buildings have been built with damp-proof courses, yet there are mediaeval buildings which have stood for hundreds of years. You need to achieve a balance between water entering and leaving the walls:
Moisture enters the wall from three directions:
Most houses of the period have a slate damp proof course. This prevents ground water passing further up the wall.
Moisture can travel through the wall through any porosity in the bricks, and along the mortar beds. So it can track through to internal walls.
Moisture leaves the wall by evaporation, either into the internal air or outside, and by the air flow through air bricks. Expect a saturated wall to dry at one month per inch of wall i.e. an eight inch wall will take eight months to dry out.
Our objective is therefore to manage this balance. We can do many things to improve it:
There is inevitably a trade-off here; we want to have a warm house but we don't want damp walls.
A 'french drain' is a useful method for drying the base of walls. Dig a ditch 30cm deep and 30 cm wide. Paint the bottom liberally with rubber compound or bitumen, and paint the wall with it too up to about 8cm below the top of the ditch. Make sure that the drain runs away around the house in a smooth run to a soakaway or at a rainwater drain. By painting with rubber or bitumen you will stop the water soaking through at any low spots in the drain. Check the water runs away through this drain by using a hose or buckets of water. Then make a wall to the side of the trench away from the house wall with old slates and then fill with gravel. Avoid walking on the gravel as this will compress the gravel and may puncture the membrane.
More drastic solutions are:
Basements require special care. Never use impermeable paint or paper, or limewash (which attracts moisture); instead use a porous emulsion, undercoated and mixed with an anti-fungal treatment.