Scrub the grapefruit and lemons. Chop the grapefruit in half, then cut each half into 4 segments. On a board, slice the sections thinly - any juice that escapes should be collected and added to the pan. Reserve any pips and the triangular end pieces of the peel. Put the sliced grapefruit into a preserving pan or large saucepan. Squeeze the lemons.
Gather up the triangular end pieces of peel, the pips and the squeezed lemon halves in a muslin cloth or a new superwipe cloth. Tie securely with string.
Add the lemon juice, water and bag of peel to the pan.
Bring the fruit to the boil. Lower the heat and simmer gently, uncovered, for 1 1/2 hours or until the peel is tender - at this point it will disintegrate when pressed between thumb and forefinger.
At this stage the liquid should have reduced by almost half. Remove from the heat, take out the cloth and, when it's cool enough, squeeze it over the pan to extract all the juice.
Add the sugar to the fruit and stir over low heat until dissolved. SETTING POINT Bring to the boil and cook rapidly until setting point is reached - about 10-12 minutes. The most accurate method of testing for setting point is by temperature. Stir the marmalade and hold a cooking thermometer in the centre of the boiling marmalade - you may need to hold the thermometer with a cloth. When the temperature has reached 105 ºC, it is at setting point.
SAUCER TEST Alternatively, try the saucer test. Turn off the heat. Spoon a tablespoon of the marmalade onto a cold saucer and leave to cool for a few minutes. If it frills or wrinkles when the surface is pushed with a finger, it is at setting point. If it hasn't reached setting point, return to the heat and boil for 1-2 minutes and test again. Repeat until setting point is reached.
FLAKE TEST Another method is the flake test. Lift some marmalade out of the pan on a wooden spoon, allow it to cool a little and then let it drop back into the pan. If it is at setting point, it will run together along the edge of the spoon and break off in flakes.
BOTTLING If there is a lot of scum, stir in a good knob of butter. Leave to cool about 15 minutes, then stir and pour into clean warm jars, filling them right up to the very top.
COVERING Press a waxed disc, waxed side down, on the surface of marmalade. Leave to cool and then cover with a lid, or cover immediately with a cellophane disc. Moisten top side of the disc, then stretch over the top and secure with elastic band.
Makes 2,4-2,5 kg ± 5 x 500 ml jars.