Damp treatment in listed buildings is likely to require consent. It is important to seek the advice of an independent qualified surveyor rather than rely on a quotation from damp proofing company. In the first instance, the cause of the damp should be identified, and as far as possible removed. Chemical based damp treatments and standard ‘tanking’ systems are not always effective in historic buildings; particularly where these are constructed in flint or bungaroosh, and are rarely appropriate. They do not allow the building to breathe as it was originally designed to do. They can trap in damp and actually increase the rate of decay, and may also just move the problem elsewhere. Such treatments are also irreversible. Dry lining systems may be more appropriate in historic buildings. These should be ventilated.
The Society for the Protection of Ancient Buildings (SPAB) have produced some useful guidance for home owners on treating damp problems in historic buildings. This guidance can be found here [10mb, PDF]