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Put two small saucers in the freezer – you will use these to test the setting point of the jam.
Put the fruit in a large saucepan and pour over 200ml water. Peel 3 or 4 strips of zest from the lemon and add to the fruit. Bring to a simmer and cook gently for 15 mins until the fruit has softened, stirring occasionally. Remove the stones from the pan as they start to come to the surface – some may need a little help detaching from the fruit. It’s a good idea to count the fruit in and stones out! Once all the stones are removed, remove the lemon zest strips.
Add the sugar and juice of half the lemon to the pan, and stir until the sugar has completely dissolved. Turn up the heat and bring to the boil, and cook for 10 mins, stirring often to ensure it doesn’t catch. After 10 mins, take a saucer out of the freezer and spread a teaspoon of jam over it. Let it sit for 1 min before pushing your finger through the liquid. If it starts to wrinkle, the jam is ready. If not, return to the boil and check again after 2 mins. Continue to do this until the jam reaches setting point (105C on a thermometer).
Allow to cool a little, then pour the jam into sterilised jars and allow to cool completely. You can sterilise the jars by washing them in hot soapy liquid, rinsing and placing in an oven at 180C/160C fan/gas 4 for 5 mins. Remove carefully and allow to cool. Store in a cool, dark place.