Many thanks for that. I can’t just board over the plaster then, if I remove the mould first? Plaster seems solid enough.

 

Problem is washing down the mold isnt always a success. Best practice is to remove affected plaster and either re-plaster or new plasterboard. Removing plaster will also allow you to see brickwork, which will tell you whether moisture is coming in from outside or its just ventilation issue.

Plasterboard over the top is just masking the issue.

Try washing down, with mold remover. Then use a small fan to help circulation of air in the room. If problem stay clear for months then it was just ventilation issue.

Edit another option, is to hire or buy a decent damp meter, with long probes ,. drill into the plaster and partway into brick work. Check level of moisture content. Do some comparison test holes in rooms where no evidence of mold occurs. These holes are only 5 mm diameter usually (on my meter) and about 30mm apart. If you have low level of moisture deep in the brick, it points to ventilation problems. I have such a meter, for testing flow screed floors. They're an invaluable tool to ascertain how well a new build is drying out before moving onto second fix and laying finished tile/wood/carpets

 

Thanks Andy, good to know, especially re damp meters. She’s gonna get a radiator installed and has a small window for ventilation too.