Peel and core the apples and cut them up. Cook in very little water until soft. Wash the plums and cut them up, taking out the stones. Add the plums to the apples and cook until soft. Sieve and weigh the fruit, and add 12oz/375g sugar to each 1lb/500g fruit pulp. Stir the sugar into the fruit over low heat until the sugar has dissolved. Boil until thick and creamy. Pour into hot jars and cover.