Course it's not exterior, it's in the kitchen
- yeah, it's an exterior wall....
I had a feeling people were going to start talking about damp...
To answer all the others bits...
Fairly sure it dried out normally, there was a space of several weeks (due to MFI losing our kitchen) between plastering being done and any painting.
Unsure about the mix quality, but as it's only happening in one place (that I know of) - hopefully not.
I think it did start at the bottom, but can't be 100% sure on that.
Floor was replaced as part of the refit.
As far as I can recall there are no pipes behind there.