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Redoing wth lime plaster is the "best" way to do it, but it's a messy job, and not that many plasterers do lime.
Out of interest why this specific type of plaster - i.e. what kind of property does it have that others do not?
Howard-
It breathes. For old houses built without cavity walls lime plaster on the inside, lime mortar for the bricks and lime render for any rendering will be better than cement-based products
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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.
bq
NotThamesWater
Be wary of any of my recommendations - I'm just an amateur at this stuff.
I've stripped one of ours back to brick and installed an insulated stud (all because of damp).
Redoing wth lime plaster is the "best" way to do it, but it's a messy job, and not that many plasterers do lime.
Another room isbduento be taken back to brick, but only to waiste height, as we're cupboarding it.