Condensation has to be considered as a likely cause, of course, but the history of the house and the fact that the damp is clearly emanating from one corner would suggest that this probably isn't the main reason for the damp.

Three things suggest that it probably isn't condensation: 1. The problem has only occurred since about 2014 (possibly the exceptionally wet winter of 2013-4) and the same tenant lived in the house for ten years from 2009. He was pretty assiduous at reporting potential problems and would almost certainly have reported the damp earlier if it had been noticeable 2. There is evidence of damp on the outer skin (moss growth) and in the cavity (the sand I scooped out at the bottom was damp) and 3. The internal damp was clearly emanating from one corner and there's very little evidence of condensation even in the bathroom so I think the tenant was pretty attentive.

So I suspect it's penetrating damp - caused by an overflowing gutter, poorly sealed cable inlet or pooling in the drainage channel abutting the front wall. I've hopefully eliminated the first two now, and if the problem persists I'll need to take this up with the council as they constructed the channel.