Put the sugar and 100ml water in a medium saucepan over a low heat, and stir now and again until the sugar dissolves. Don’t let the syrup boil.
Meanwhile, tip the strawberries into a food processor and blend to a purée. Pour into the sugar syrup with the mint leaves and turn up the heat. Simmer for about 5 mins until the mixture thickens slightly. Once thickened, strain through a fine sieve to get rid of any seeds and leaves. Allow to cool for 15 mins.
Pour into a container and freeze for 1 hr, then rake the top layer with a fork and put back in the freezer. Repeat every hour for the next 4 hrs until the mix becomes a crystallised slush. Serve in glasses with a sprig of mint on top.