Wash the fruit and cut in half. Squeeze out the juice and tie the pips and pith in a muslin bag. Shred the peel finely and put into a preserving pan with water and muslin bag. Leave to soak overnight. The following day simmer gently for about 11/2 hours or until the contents of the pan are reduced by about one third. Remove muslin bag, squeezing gently to extract as much pectin as possible. Stir in the sugar and cook over a low heat until dissolved, then boil rapidly until setting point is reached. Leave to cool for about 10 minutes so that the peel is evenly distributed, then pour into hot sterilized jars. Cover tightly.