1 pink or red grapefruit
4 blood oranges
3 Seville oranges
150ml lemon juice
1kg unrefined granulated sugar, warmed in a low oven

The grapefruit and oranges should weigh 1kg in total.
Wash the fruit.
Peel the grapefruit with a vegetable peeler and cut the skin into fine shreds.
Cut all the fruit in half and juice it.
Put the juice in a large bowl.
Add enough water to bring up the volume to 1.25 litres. Scoop the flesh out of the grapefruit and tie it in a muslin bag.
Add the bag to the bowl.
Shred the orange skins as finely as possible and put them into the bowl along with the grapefruit rind.
Leave overnight to soak.

Cook this mixture the next day until the shredded skin is completely soft. It will take about an hour and a half.
Strain the jam through a sieve (keeping the shreds) and measure the liquid, making sure you squeeze all the pectin-filled juice from the bag.
You should have about 750ml. Top up with water or reduce if not.
Discard the bag.