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The plaster went in quite a while ago, probably 2 months now so has been dry for a while. The windows only went in just before Christmas and I’ve been tiling gradually though so it’s been pretty inconsistent on moisture levels I think. Hopefully it’s down to excess moisture and cold though, this is sounding likely from the other comments too, so going to do our best to warm it up a bit (radiators aren’t going in til end of January so just got a little electric heater up there now) and see what difference that makes.