Any legal boffs in the room?

Also worth checking if there is an old unused fireplace with a safe mounted into it which is full of gold.

I'm not absolutely sure but the offending wall in my house is flat, there is no chimney breast. Also, the chimney stack only has one pot on it and this is central to the stack. You'd expect that if it served more than 1 fireplace that there would be more than 1 chimney pot and obviously if there were more than 1, perhaps one had been blocked off, this would be to the side of the stack, rather than central. 

I think you have a brick house mindset. With stone buildings the chimneys tend to be integral with the walls. We live in a stone house, originally there was a chimney and fireplace in each room. In the dining room and living room there are visible fireplaces with the giveaway of small alcoves, but in the bedrooms the fireplaces are all removed with no visible sign except a small alcove on one side in 2 of the 4 bedrooms. In those rooms with no fireplace and no alcove, there is no obvious hint that there once was a fireplace.

I thought it was a typo initially.

Reckon that's the route I would go down, not worth the bother.

Seems sensible..

I’ll mention another thing which is that if your house is anything like ours, there is an outer shell of stone, and an inner shell of timber / lath and plaster etc rooms. There is a gap between the inner and outer shell, which has to be ventilated. The timbers are pegged into the stone wall of course, but otherwise they must not be touching. This is because there is no damp proof course, and stone can be quite porous,

What can happen is that decaying lime mortar crumbles and little bits of sand-like debris fall down between inner and outer walls. Sometimes this gets trapped and builds up such that it bridges the gap between the stone and the wood, and moisture gets into the plaster. The travelling moisture also carries “salts” which contaminate the plaster such that, even after the bridging issue is fixed, the plaster remains stubbornly damp. This is because the salts make the plaster hygroscopic - it literally pulls moisture out of the air.

So in summary, if you have access to the loft space I would check at the edge of the timber structure where the damp is, look down and see if there is any debris bridging between stone and wood.

I had this problem in our previous granite house, and have a similar problem in this house under a very slight leak from a roof light window. Just a few drips with very heavy rain and wind, but over the years it’s made a patch of the plaster in the ceiling of the room below, hygroscopic such that it is permanently damp despite the leak long since having been fixed.

It can seem that these sort of damp problems are linked to rainy weather and thus indicative of a leak, but in fact it is down to the increased air humidity during wet weather.