EVEN in these days of restaurant moals and food in packets, every self-respecting housekeeper makes a point of celebrating Christinas with the produce of her own kitchen, and Christmas cake is one of the easier things to make. Mrs. Cottington-Taytor. director of the Good Housekeeping Institute, will give some good advice as to how to concoct that thick, rich, fruity mixture that has been responsible for so many bad dreams on Christmas night.