1. Halve and squeeze the juice and pips from the grapefruit and place in a large mixing bowl with the lemon juice. Slice the peel any way you like: my new favourite way is using the slicing attachment on the food processor, which quickly turns the peel to fine slivers. Add the peel and water to the juices in the bowl, cover and leave for 24 hours.

2. Tip the contents of the mixing bowl and the cardamom pods into a large saucepan. Bring to the boil and cook until the peel is very tender – just over an hour for very thin shreds or two hours for thicker pieces. The contents of the saucepan should have also reduced by half.

3. Test for pectin and acidity (see tips), and reduce volume more if needed to concentrate the marmalade for a good set. Add the sugar, bring to a bare simmer then switch off and leave for 24 hours. This allows the peel to slowly absorb the sugar syrup and produces a better peel texture and transparency.

4. The following day have your clean jam jars and screw lids ready. Bring the marmalade to a rolling boil. Skim off any pips that rise to the top and cook until the temperature reaches 104C on a thermometer. Switch off the heat, leave until the temperature drops to 80C then ladle the marmalade into the clean jars and screw on the lids immediately.

Serve with homemade buttered crumpets.

World Marmalade Awards judge Dan Lepard shares his marmalade tips.