Wash the fruit and cut in half. Squeeze juice and tie the pips in a muslin bag. Mince peel coarsely. Put minced peel, lemon juice, water and muslin bag into a preserving pan. Simmer the mixture gently for about 2 hours or until the peel is tender and the contents of the saucepan are reduced to about half. Remove the bag of pips, squeezing gently to remove as much pectin as possible. Stir in the sugar and cook over a low heat until the sugar is dissolved. Bring to the boil and boil rapidly until setting point is reached. Remove from the heat and stir in the butter; this helps to reduce the scum. Leave to cool for about 10 minutes so that the peel is evenly distributed, then pour into hot sterilized jars and cover tightly.