Wash the fruit, then halve and squeeze the juice into a preserving pan or large heavy-based stainless steel saucepan. Catch any pips and membranes and place in a muslin bag. Cut the peel into thin shreds and then add to the saucepan with the muslin bag and 2 litres of water.
Simmer gently, uncovered for about 2 hours or until the contents of the pan are reduced by half and the peel is tender. Remove the muslin bag, leave to cool, then squeeze any liquid from the muslin back into the pan.
Add the sugar to the saucepan and bring to the boil. Begin testing for the setting point by placing a drop of marmalade on a cold plate. If it wrinkles when you push it with your finger, it is ready.
Transfer to sterilised jam jars and leave undisturbed for 8 hours in order to achieve a good set.