From reading several instances online, my feeling is that the concrete slab in the rear room diverted moisture that would otherwise rise up through the floorboards, into the surrounding walls, hence the damp line on the walls and plaster. The lime and red brick that comprises the inner walls is quite damp to the touch:
There is the remainder of a pipe coming from the street, up through the soil in the below picture. Not quite sure what it is yet:
Below the fireplace in the front room:
This excavation has filled a 14 cubic yard skip, a larger one is arriving tomorrow!