Mould is formed when there is a build up of damp static air. That is why it is worse in the back of your wardrobe - it is colder and the air doesn't move. The moist air comes from your house - water vapour in the air from cooking, airing clothes, showering - it doesn't all get extracted - it then rises up through the plasterboard and into the loft space where it condenses as water (because it is colder once it has passed through your insulation layers on the ceilings.
Do you have vents (in the form of flymesh) at the edges of the eaves? - these are designed to allow air into the cold roof space and ventilate it. Are they blocked with insulation? In extreme case it is sometimes necessary to fit a vent at the line of the apex of the roof - to let the moist air out.