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Wash your jars and lids in hot, soapy water, rinse, then place on a baking tray and put in a low oven for 10 mins or until completely dry. If you want to use rubber seals, remove the seals and cover in just-boiled water. Make sure you sterilise any funnels, ladles or spoons you’re going to be using too.
Put the fruit into a jam pan or a large, wide, heavy-based saucepan. Leave the stones in (see tip below). Add 150ml water and bring to the boil. Put a couple of saucers in the freezer. Lower the heat and simmer for 15 mins or until the fruit is soft.
Tip in the sugar and stir over a very low heat until the sugar has completely dissolved, up to 10 mins. This step is vital – if you don’t dissolve the sugar, the bottom of the pan may catch and burn. Raise the heat, bring to a full rolling boil, then rapidly boil for 10 minutes. Don’t stir until the setting point of 105C is reached. If you don’t have a thermometer, test the jam by spooning a little onto a cold saucer. Wait a few seconds, then push the jam with your fingertip. If it wrinkles, the jam is ready. If not, cook for a few mins more and test again, with another cold saucer. Once you have reached 105C or setting point, stir the jam thoroughly.
Remove from the heat, skim off any excess scum, then stir a knob of butter into the surface (this helps to dissolve any remaining scum). Leave for about 15 mins so the fruit can settle – if you decant the jam too soon, all the fruit will sink to the bottom. Pour into sterilised jars, label and seal.