Thanks for all the comments!
I have taken off all plaster and it seems to be drying now, surprisingly fast...
I think that the plaster, the pre owners have put on as well as the paint, sealed the wall completely for like 40 years.
I just registered on this forum and thought I give it a try as I am really struggling at the moment.
I own an old cottage in Dorset which I’m renovating at the moment.
The pre owners haven’t used the cottage for 30 years. So it has not been heated at all.
every single wall is fine...