Wash the fruit, cut in half and squeeze out juice. Remove as much of the pith as possible and put in a muslin bag with the pips. Shred the peel. Put the peel into a preserving pan with the juice, water and muslin bag. Bring to the boil and simmer gently for about 2 hours or until the peel is tender and the liquid is reduced by half. Remove the bag of pips, squeezing gently to extract as much pectin as possible. Add the sugar and treacle and heat gently until the sugar has dissolved. Bring to the boil and boil rapidly until setting point is reached. Remove from heat and allow to stand for 5–10 minutes so that the peel becomes evenly distributed. Pour into hot sterilized jars and cover tightly.