Wash the fruit.
Dry and cut the zest off the grapefruit.
Slice the zest thinly and put into a large heavy-based pan.
Squeeze the juice form the grapefruit and lemons and add to the pan with 3,75 litres of water.
Slice the lemon skins and pith and place in a muslin bag, together with the grapefruit seeds and sliced pith.
Add the bag to the pan and bring the liquid to the boil.
Cook for about 30 minutes until the skins are tender.
Remove the bag and squeeze over the pan to extract the pectin-rich juice. Leave to stand overnight.
Bring the mixture to the boil and add the sugar and ginger to the pan.
Remove the pan from the heat and stir until the sugar dissolves completely, then bring the mixture to the boil, cooking rapidly for about 15 minutes or until it begins to gel.
Remove from the heat and allow the marmalade to stand for 15 minutes before pouring into clean, dry, warm jars that have been sterilised.
Makes about 6 jars.
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