Dice the watermelon (about 2 cm), remove seeds and weigh out 450g.

Place watermelon, half the lemon juice (2 tbsp) and 2 tbsp water into a preserving pan or similar.

Bring to a boil, then reduce heat and simmer under a lid for 10 – 12 minutes. (Stir a couple of times and don’t let the watermelon dry out). This will cook the watermelon but it won’t soften.

Transfer the watermelon to a bowl or blender and blend until smooth (I use a stick blender).

Pour the puree back into the pan along with the rest of the lemon juice (2 tbsp) sugar and optional white rum (1 tbsp) if using.

Heat gently (do not allow to simmer) and stir until the sugar has dissolved completely. Check for any grittiness with a wooden spoon.

Turn the heat up and bring to a rolling boil for 2 minutes.

Remove from heat and place a few drops of jam on a chilled plate then pop into fridge for a minute.

Test for set by pushing a finger through the jam, if it forms a subtle crinkle and is tacky and gel like then it is ready. Mine was just ready at 2 minutes but I probably could have done another 30 seconds for a firmer set but it would be easy to go overboard and make toffee so do be cautious).

If not, bring to a rolling boil for another 30 seconds (yes, this jam is so quick to reach setting point) and test again until setting point reached.

Pot up into hot sterilised jar/s and seal with lid/s immediately and allow to cool. (A jam funnel is useful here but not imperative).

Store in a cool, dark place, I like it straight from the fridge!

Yield 400 mls (1 large or 2 small jars).