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Tip the grapes into a large saucepan set over a low heat, then cover and leave to gently cook for 5 mins until the juices start to run. Take a potato masher or fork and mash up the grapes. Leave to cook for about 10 mins more, mashing every now and again until the grapes are falling apart. Place a clean tea towel or kitchen cloth in a sieve set over a bowl, then pour the grape mixture into this. Let the mixture drip through for at least 1 hr or preferably overnight.
Measure out the juice (you should have about 600ml) and pour it into a pan along with the sugar and lemon juice. Set the pan over a high heat and bring to the boil. Skimming any scum as it boils, let the mixture bubble until the temperature reaches 105C on a sugar thermometer. If you don’t have one, put a small plate in the freezer for 5 mins, then pour a little of the juice onto the cold saucer. After 1 min, run your finger through; if the jam wrinkles slightly, it’s ready. Pour the hot jam into a sterilised jar. Will keep unopened for up to 3 months.