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From this it sounds like the damp might be coming from the neighbours kitchen /utility guttering behind the wall?
rj-
It is rank in there. He's quite old and I help him clear stuff out from time to time, so I see lots of piles of stuff. What could I do about that if so and how could I confirm? He's not going to be interested in doing anything himself. Presumably I'd want to block it off at the stone:
Clean back the stone (the entire wall or just the currently affected area?) so there's minimal loose dusty stuff.
Treat the stone (pva?)
While I'm there put in some damp injection cream near the bottom of the wall to be sure.
Wait for a while (how long?)
Fill it then skim or (or plaster board somehow (maybe screw in battens into the stone (though presume screwing would wreck the damp proofing?)
None of the above?
????
Profit?
CYOA
It's around 1900 but I'm sure that section of the room was extended at a later date. Most of the house is a mix of brick and stone but the back of it appears to be cardboard. It's a wall that joins a neighbour but his house doesn't go that far back. His kitchen is about half the size of ours and it goes into his utility room (still inside though).