Damp patches in corner of first floor room

From your description it does sound like condensation, it’s probably a cold point of the room which the moisture is attracted to, I would have someone look at the plaster as it may need replacing as it’s been water damaged due to being damp, I would also using a paint called warm wall, it’s from a supplier called Pam ties, available online, this raises the surface temperature and also has anti mould properties, I hope this helps

I would suggest the issue relates to condensation it would be advisable to test the moisture level on the plaster coat. you may need to strip back the plaster if its defective and renew.

It’s definitely condensation as mould doesn’t grow on damp walls. Make sure you have good ventilation. ie bathroom fan is on, and runs on for at least 15 minutes after you have finished. Also the kitchen extractor is on when cooking. Make sure you open your windows to allow for good air change, leave them slightly open if not in but locked. Hopefully this helps if not further investigation may be needed.

if your suffering any form of mould on cold walls in cupboards etc, then in line with treating the mould you need to ensure you ventilate the rooms that make the moisture ( the moisture producing rooms) bathroom kitchen, and utility rooms to stop steam from spreading throughout the dwelling as wiping of mould, apply mould paint is only a repair it's not a cure, fitting good humidistat fans to increase air flow is the cure in about 80% of cases, as well as not drying clothes in doors, changing your heating patten and opening windows a couple of f times a day to allow dry fresh air in and the damp used air out.