Wash the fruit, cut in half and squeeze out the juice, reserving the pulp and pips. Scrape all the white pith away from the skins. Put the pith, pips and pulp into a bowl with 2 pints of water. Shred the peel finely and place in another bowl with 4 pints of water, and the juice. Leave both to stand overnight. Strain the pips, etc., through a muslin bag and tie it loosely. Put the strained liquor, bag, peel and juice all together in a preserving pan, simmer until the peel is tender. Remove from heat and discard the muslin bag without squeezing. Add the sugar, allow to dissolve completely over gentle heat, then boil rapidly until setting point is reached. Remove from the heat, cool for a few minutes, then pot and cover tightly.