Hi Mark As you say, a real problem this time of year. The fact is, that if as bad as you say, you have to deal with the cause. things like drying washing indoors, cooking with no extractor fan, etc. can put so much moisture in the air that one quick shower, and there's mould everywhere. Have you any extract fans ? what insulation do you have ? Is the heating sufficient ? Yes there are anti fungal paints, and paint additives, but, in a case as bad as this, there's very little chance of any improvement. It's down to the cause, I'm afraid.