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I doubt that your damp problem is caused by the mixing of different materials.
There could be many different reasons for it.
But that is not one I've heard of.
wildwest-
In at least one place it is -rather, it's a combination of factors, one which is mixed materials - cement render over lime render, sitting on a base where water sits (because the path was laid with waterproof adhesive over concrete), so water wicks up, then bridges the join between a modern cavity wall and solid old wall, hits old plaster again, and blows out the new more waterproof plaster laid over the top.
It's going to be fun fixing this lot.
In other news, I'm going to have a go at installing a lead flashing apron later.
NotThamesWater
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.