Peel and core the quinces and cut the flesh into small cubes. Put into a pan with just enough water to cook without burning. Simmer until the fruit is soft but unbroken. Weigh the fruit and liquid and allow 1lb/500g sugar to each 1lb/500g fruit. Stir in the sugar and leave overnight. Bring to the boil and then simmer for 20 minutes. Pour into hot jars and cover.