As others have said, try to identify the source of the damp before laying final floor coverings. Do you have any hot water pipes or heating pipes in this area? If you have the usual hot water circuit for the time (with hot water from an indirect cylinder heated by your boiler), then a small leak may not show immediately as water flowing through your meter. The vagueness of float valves in storage tanks means that small quantities of water can be drawn without make up water being immediately needed to replace it.
I have a 1973 property of similar construction to yours. When we moved in about 10 years ago it was discovered that a damp patch was not due to a dodgy patio door, but down to a fractured elbow on a hot water pipe that had clearly been leaking for some time. This had caused rotting of some skirtings and door frames. The damp was just at a level where an inspection didn't show anything too alarming, but once in, the effects were found to be quite wide spread. Water had tracked under most of the original tiled floor covering, so this has all been removed.
One issue with pipes laid in solid floors is that a leak can develop, but the resulting damp may creap along the pipe channels and affect other areas.
If you don't have a good handle on pipe routes, I would try to establish these as it may help pinpoint the source of the problem.