Cut the oranges and lemons in half, squeeze the juice and strain it into a stainless-steel or enamel preserving pan. Using your fingers, remove all the ﬂesh and pips from the squeezed fruit and tie the pips securely in a muslin square with the halves of lemon peel.
Cut the orange peel pieces in half, then crossways into strips about 5 mm thick. Add the strips and the muslin bag to the pan, pour in the water and bring to the boil. Reduce the heat and simmer for 2 hours, or until the peel is very soft and the liquid has been reduced by half.
Remove the muslin bag from the pan, put it in a bowl and leave until cool enough to handle. Squeeze the bag to remove as much juice as possible, then pour the juice back into the pan. Discard the bag.
Add the sugars and treacle to the pan and stir over medium heat until the sugar has dissolved. Bring to a full rolling boil and boil for 15–20 minutes. Remove from the heat and test if setting point has been reached.
When the marmalade will set, skim any scum from the surface. Leave the marmalade to stand for 20 minutes to allow the peel to settle. Stir to disperse the peel evenly, then pour into clean, warm, dry jars and cover the jars with waxed paper discs and cellophane covers. When cold, label and store in a cool, dark, airy cupboard.