The Wickes leaflet also recommends insulating the underside of the
roof, by use of polystyrene blocks in between the roof timbers. They
also sell aluminium foil stuff for the same job.

Wickes charge =A313 for a roll of aluminium foil insulation to staple
onto the underside of the roof. At those prices, it would cost quite a
bit to cover the whole roof...probably more than I would save on
heating.

Bruce