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Put the fruit into a preserving pan or large heavy-based saucepan. For blackberries, add 50ml of water and 1½ tbsp of lemon juice; for plums (halved and stoned), use 150ml of water; for strawberries, add 3 tbsp of lemon juice (no water); and for raspberries, add nothing. Bring to the boil.
Lower the heat. For blackberries, simmer for 15 minutes; for plums, simmer for 30-40 minutes; for raspberries, simmer for 2 minutes; for strawberries, simmer for 5 minutes. The fruit should be soft.
Tip in the sugar, stir over a very low heat until the sugar has completely dissolved. Raise the heat, bring to a full rolling boil, then rapidly boil blackberries for 10-12 minutes, plums for 10 minutes, raspberries for 5 minutes or strawberries for 20-25 minutes - don’t stir though – until the setting point of 105C is reached.
Remove from the heat, skim off any excess scum, then stir a knob of butter across the surface (this helps to dissolve any remaining scum). Leave for about 15 mins so the fruit can settle. Pour into sterilised jars, label and seal.