The mould is most likely to be caused my condensation which is rising as humidity from the main living area. As humidity is relative, warm air will hold water, and cold air will not. Therefore as the humid air reaches the loft space then the water within the air will be released and condense on the roof timbers. The best way forward is to ensure that the humidity within the main living areas remain at an acceptable level by the correct use of ventilation.
Lofts are usually quite well ventilated therefore additional ventilation to this area will probably not make a lot of difference. However is is wise to ensure that the existing insulation is not blocking the eves and there is air passing through.
Peter Barber CSSW. CRDS