Step 1

Cut all the grapefruits in half and squeeze the juice from each through a fine sieve into a bowl and reserve, keeping any pips. Cut the grapefruit halves again into quarters and, if the pith is very thick, carefully remove some until the rind is ½cm-1cm thick. Put any trimmed pith and pips into a clean muslin cloth and seal tightly with string or a rubber band to make a bag.

Step 2

Slice the remaining grapefruit rind into 1-2mm slices. Put the sliced rind and muslin into a large pan with 1.5 litres of water, bring to the boil and simmer very gently for 2 hours until nearly all of the water has evaporated.

Step 3

Remove the muslin bag from the pan and use a pair of tongs to squeeze out the pectin-rich liquid back into the pan with the rind. Put a small clean plate in the freezer.

Step 4

Pour the sugars and the reserved grapefruit juice into the pan and bring back to a gentle simmer to dissolve the sugar. Put a sugar thermometer into the pan, turn up the heat and boil for 15-20 minutes, stirring regularly to avoid it catching, until it reaches 105C.

Step 5

Remove the plate from the freezer and spoon a little of the marmalade onto the plate. Gently push the marmalade with the back of a spoon – if it’s at the right setting point, a rippled skin will appear on the top. If no ripple appears, return the plate to the freezer, boil the marmalade for another 5 minutes, then try again. Repeat until necessary.

Step 6

Stir the lemon juice through the marmalade and pour into sterilised jars and seal. It will keep in the fridge for 6 months.