Sterilise 12 × 250 ml jars and their lids by boiling for 10 minutes or heating in an oven at 140 °C. Keep hot until ready to fill.
Whisk together sugar and pectin in a bowl,
In a preserving pan, combine plums and lemon juice. Add 75–100 ml water if fruit is very firm. Simmer gently for 12–15 min until starting to break down.
Remove pan from heat, stir in sugar–pectin mixture until dissolved.
Bring to a vigorous rolling boil and hold 1–2 min, or until jam reaches 105 °C / passes plate test.
For a firmer jam, remove from heat and let the jam cool for 10–15 minutes. Then return to the heat and boil again for about 2 minutes to lock in a firmer texture.
Skim any foam, ladle jam into the hot jars leaving about 5 mm headspace, and seal with lids.
Place jars into a large pot of boiling water, ensuring they are submerged by at least 2–3 cm. Once the water returns to a full boil, process for 10 minutes.
