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If it’s a potentially damp wall, then don’t fill with bonding plaster as it’s very hygroscopic. Use a cement mortar mix which will probably be fine, or add a little damp proofing agent if it really has an ongoing damp problem. Build it up in layers if it’s over say 30mm deep, and leave the top to be finished with finishing plaster.
rodabod
wildwest
This is where I'm a bit wary - previous builders have mixed and matched modern with old materials willy nilly, and there are problems everywhere with damp because of it.
And although this is on an internal wall, it is an internal wall where damp exists, and I don't want to exacerbate it.
I may end up stripping of the top layer of gypsum plaster (which looks like it may come off in one piece) and giving it a finishing coat of lime plaster.
Which means, I guess, I just need to fill the hole with lime plaster & hair mix, then finish with a rough skim coat.