Put 2 or 3 small saucers in the freezer; they'll be used later to quickly cool teaspoons of jam to find when the setting point has been reached.

Wash 3 jars with screw-on, plastic lined lids in hot soapy water then rinse. Turn them upside down to drain.

Put the jars and lids on a baking tray and place in the oven at its lowest setting.

Turn off the oven when the jars are very hot, but leave them inside until you're ready to pot the jam. The jars must still be hot when potting up.

Remove and discard the peel, pith and seeds from the grapefruit. You can do this by cutting into quarters then using a knife to cut out the flesh from between the membranes. Do this step over a bowl to catch all the juices.

Weigh the fruit and juice then weigh out the same amount of jam sugar.

Put the fruit, juice and sugar in a large, wide pan and place over medium heat.

Stir until the sugar dissolves then turn up the heat and bring to a boil (you can put on a lid to speed this up).

As soon as it comes to a boil, take off the lid and boil hard until setting point is reached (10-15 min).

Testing for setting point

Take the saucepan off the heat.

Flake test: Take some jam on a spoon, cool it a little, then let it fall off the edge of the spoon. If the final bits of jam come together into 'flakes' rather than running off in a stream, then proceed to the Cold Saucer test. If it doesn't flake, boil for another 1-2 min then test again.

Cold Saucer test: Take one of the cold saucers from the freezer, put a teaspoon of jam on it to cool then gently push the jam with your finger. If it wrinkles, then setting point has been reached. If not, boil for another 1-2 min then test again.

(Optional) Using a jam thermometer: Jam is said to set at 104.5° Celsius. However, various factors (e.g. the amount of pectin present) can affect this so I don't recommend using a thermometer only.

Once setting point has been reached, take the hot jars and lids from the oven and pour the jam into them (use a jam funnel or transfer to a clean heatproof jug and pour from there).

Protecting your hands, put the lids on and leave to cool. When just warm, you may be able to tighten the lids a little more.

When the jars are completely cold, check for an airtight seal by pressing down in the centre of the lid.

If you can't press it down, or the centre does not pop back up again, then you have an airtight seal. If the centre pops back up: see Recipe Note #3 for what to do.

Ready to use immediately but unopened jars should keep at least a year in a cool, dark place.

Once open, store in the fridge and use within a few weeks.