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Put the sugar and 270ml water in a saucepan over a low heat and stir until the sugar has dissolved. Raise the heat and simmer for 5 mins, or until the liquid has become a syrup. Set aside to cool.
Put the raspberries and lemon juice in a food processor and blitz until smooth. Strain through a fine sieve into a bowl and discard the seeds. Combine with the sugar syrup, then pour into freezer-proof container.
Freeze for 1 hr 30 mins, then whisk with a balloon whisk or a fork to break up any ice crystals that have formed and return to the freezer.
Keep mixing the sorbet once an hour for 4 hrs to break up the ice crystals. Stop mixing when firm but scoopable. Will keep in the freezer for up to a month. Serve with extra raspberries, if you like.