Pare the rind thinly from the fruits and cut into fine shreds. Halve the fruits and squeeze out the juice, then put the juice and rind into a large pan with the measured water and ginger.
Chop the fruit halves, including the pith, and tie the pieces in a muslin bag. Add this to the pan and slowly bring the mixture to the boil. Reduce the heat, cover the pan and simmer for 2 hours or until the ginger and rind are completely tender. Take the pan off the heat and leave to stand until the muslin is cool enough to handle, then squeeze all the juices back into the marmalade and discard the bag.
Add the sugar to the pan and cook over a low heat, stirring continuously, until the sugar has dissolved. Increase the heat and bring to the boil, then boil hard to setting point. Using a slotted spoon, carefully skim off any scum, then leave the marmalade to stand for 15 minutes to allow the fruit to settle.
Stir well, then transfer to warm, dry, sterilized jars (see