I rent out a house, and mould appeared 2yrs last winter in the corner of the bedroom (top of stairs as it is open plan bedroom). I had the roof replaced, and summer & winter with new tenant was fine, but it has reappeared. My current tenants dry their clothes inside, and have said that they do not use all the storage heaters (to cut their heating bill), I am inclined to think that it is lifestyle living, but as a landlord I have a responsibility. The house is situated on the top of a hill so is exposed to the elements, and we have had a lot of rain recently.
It will mainly be down to the occupiers and their lifestyle - it's in your best interests and their best interests for them to accept this - the causes are...
Moisture in the internal atmosphere
Lack of adequate ventilation
Cold external walls
Condensation forming when warm wet air meets cold walls
Condensation not being wiped up
This Scottish paper may be useful to both of you...
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