This is what happens after an inappropriate attempt at remedial damp-proofing
There shouldn't be any need to attempt to do remedial damp-proofing in a historic building and it is very likely to not be granted Listed Building Consent anyway. When done, it is often the result of an illegal alteration. In this short video clip, made on my phone, you will see the after-effects of it on a stone wall. It has clearly failed to work and actually made matters worse. The dry patch is where a waterproof additive has probably been added to a cement render under the plaster. This will now need to be carefully removed, to allow the building to dry naturally through evaporation of the residual moisture by heating and ventilation. It can then be replastered using a breathable lime plaster. Always seek specialist advice from someone with skilled expertise in building conservation before wasting your money on damp-proofing.