I had nearly identical damp caused by two issues - damp coming from the roof down through the walls to the bottom of the house (yep) AND cavity wall to outside of house filling up with debris/crap and allowing damp to pass in at ground level from outside.

Paid someone to treat it by injecting a damp proof foam into cavity wall. Been fine since.

Get a damp proofer to come and investigate and they can tell you. It's money well spent.