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Here's the theory that I've decided is trufax
Solid walled houses have soft bricks & lime mortar (in general), which are more permeable to and absorbent of water & moisture in the air.
Lime plaster is also relatively permeable (particularly when compared with gypsum plaster), allowing the wall to dry out.
A lot of the problems appear to be due to mixing old with new though - Gypsum on lime allows the lime to wick water really effectively, making it difficult to see where any damp is actually originating.
If you put gypsum onto old brickwork, it won't come off nicely either, and can damage the brickwork.
Externally, lime render does much the same - exchanging waterproofing for permeability and the ability to dry out. All cement / other render does is to crack (over time) and allow water to gather, and damage brickwork.
NotThamesWater-
Interesting, thanks. House we are after has two spots of 'damp' or whatever on the ground floor. I'm putting it down to a couple of contributing factors* and at some point we'll get it fixed should the purchase actually ever happen.
Also @bq - thanks.
* shit drainage set up on the extension, laminate floorings
Howard
Out of interest why this specific type of plaster - i.e. what kind of property does it have that others do not?