Pare off the grapefruit rind into fine shreds with a potato peeler. Halve it and squeeze the juice into a preserving pan. Chop the pith and flesh, place in a muslin bag with the pips.
Cut the lemon in half, squeeze the juice and scoop out the membranes, then place in the bag. Cut the rind into shreds and place in the pan with the juice, add the water and bag and simmer for 2 hours.
Remove the bag and place in a colander over a bowl, press out the liquid and, when cool enough, squeeze the bag to get out the essential pectin which will make the marmalade set. add the liquid to the pan with the sugar. Stir until the sugar is dissolved, then bring to the boil and boil rapidly until setting point is reached, - this might only take a couple of minutes., Remove any scum from the surface and cool for 10 minutes before pouring into warmed sterilised jars.