Insulating the loft door is effective, but in cold climates it makes the loft itself very cold indeed. Little of the the heat used to penetrate can now do so. If the cold water cistern is up there, as it is in most British houses, you will have to lag it , wrap it up to stop it freezing in the winter. Leave a space in the floor insulation under the cistern so that a little heat reaches it from below.
The cistern itself can be lagged with various materials. A special type of fibreglass blanket is sold for the purpose. There are tank lagging kits available that use cut-to size insulation boards to fit all standard sizes of cistern. The boards fasten together with clips that are supplied with the kit. Holes can be cut for the cistern’s pipes with a handyman’s knife.
Measure the exact position of the hole from the two nearest edges with a ruler. Cut as small and neat a hole as possible, and cut out a strip of board from the hole to the edge of the board so that you can slide the board into place over the pipe. When the board is in position, put the strip back and clip it down. Join the edges of the floor insulation to the sides of the tank insulation so that no heat seeps through the gap.