Put the whole fruit in a large, heavy pan, cover with water, bring to simmering point, cover and cook on the lowest possible heat for 6–8 hours. Remove from the heat, keep covered, and leave overnight. Next day, measure out 1½ pt / 850 ml cooking liquid into a preserving pan. Reserve the rest of the pectin-rich liquid for use in making other jams and preserves. Quarter the fruit, remove the pips, and thinly slice it. It will be very soft, but should not be too soft to handle. If it is, just chop it. You can also chop it in the food processor if you are not particular about having shredded peel. Transfer the fruit to the preserving pan. Heat gently, and when warm, stir in the sugar. Keep on a low heat until the sugar has dissolved, and then bring to the boil. Boil until setting point is reached. Pour into clean, hot jam jars, cover with waxed paper discs and then cellophane, and label the jars.