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Seems fine, what could go wrong?! :)
Looks like the worst side of mine is the original (presumed lime) plaster - based on what looks like horsehair bits to help it bind together. So I'm also not sure about infilling with gypsum plaster. It's not an external wall, so breathability isn't a big concern (although the other side of the wall is tiled for the bath/shower so could be hiding all sorts of hideousness) - I just want it to stay put!
Cheers. I'm just going to chuck lime plaster at it and rub it 'til it complies. Then call a professional.