Get a survey done by a damp specialist. We had damp come up on our homebuyers report, and got a quote of £10k for a damp proofing course. Then we had a damp specialist survey and he said we had the normal level of damp for a terraced house of that age, and it was nothing to worry about. When we sold the house it didn't even come up on their homebuyers report.
I've heard the problem is that some surveyors use surface readings for damp which can give incorrect readings.