Fit PAROC eXtra insulation from the upper side of the roof. Start heating only after the air and vapour barrier has been installed. This prevents water vapour from being unnecessarily transported through the structure and possibly condensing against the outer roof.
Inspect the condition of the old loft (insulation and structures). If the old construction and insulation are in good condition, add new insulation layers on top of it. If they are wet or mouldy , remove them.