Update for what it's worth. Taken off some skirting at the point where
the damp is most evident ie at the junction on the internal partition
wall bentween the suspended timber floor front room and the concrete
floor hall / back room. there is a blue plastic membrane sticking out
3-4 inches from under the concrete floor which may give its age away
(?). there has defineitly been a DPC as I can see the holes. the bricks
are dry but the plaster above is sodden.

took a look under the floor boards in the front room. cant see back
twoards the area mentioned above as there is an original stone
supporting wall under the floor boards blocking my view. whatever is
going on, the worst of it would seem to be coming from this area.

also looked round outside of house and existing DPC in external walls
appears to be about an inch above the house floor level