I solved the problem, and in case it is of interest to others, I'll explain here.

Last might I placed two self adhesive carpet tiles on the floor alongside two squares of foil held down with masking tape. This evening the screed under the floor tiles was damp. That under the foil was bone dry. Hence the cause of the damp is condensation.

Given that the house was treated for penetrating damp 20 years ago, I do wonder if that damp was also condensation.