With only four more days until Christmas, millions of parcels are on the move through the vast postal network of Great Britain. The G.P.O. is working overtime, aided by thousands of extra staff and vehicles to deal with the annual flood of Christmas parcels. Tonight Joan Gilbert and Richard Dimbleby take you to London's largest sorting office, the huge area known as Mount Pleasant, near Holborn, where you see how the parcels which pour in from post offices all over the world are received and dispatched.