There are two sides to being a good landlord, it's not unfair to expect reasonable tenants is it?
I agree but you haven't actually ruled out a underlying reason and are assuming the problem is of their causation. Could it not be something has changed - it has been very wet and windy since you left etc.
You are suggesting they aren't being reasonable tenants and yet you have no proof of this other than a report of mould which has not been investigated sufficiently by your or anyone else. Are the windows and trickles vents still functional, is the extractor fan sufficient and functional - all it may take it the pipe to move in the cold weather and condensation to pool there and you've got a problem, etc.
All I am saying is that I know from personal experience it is not necessarily the fault of the tenant that mould occurs and I would suggest in such an increasingly damp country it is kind of inevitable and as a landlord should be expected.
You expect tenants to keep a house in good repair but you can never expect for them to care for it as they would their own home because at the end of the day legal protection for renters is that poor in this country you should never assume a rental property is a home or anything more than a temporary residence.
When I am saying it is your problem I am directly referring to as the owner one way or another you have to sort it out - something you have sensibly done.