Place the sugar and 500ml water into a small saucepan over a low heat, stirring until the sugar is fully dissolved. Increase the heat to bring the syrup to a boil without stirring. Reduce the heat slightly and simmer, without still without stirring, for about 3 minutes. Remove from the heat and allow to cool completely.
In a blender or food processor, combine the cooled sugar syrup with 50g mint leaves and the 1 tbsp balsamic vinegar, and whizz to a liquid.
Strain through a sieve into a freeze proof container and freeze for 2 hours, until the edges start to set. Remove and stir vigorously using a fork, or place in a food processor and whizz to break up the crystals. Freeze again. Continue to break up the ice crystals using this method every hour for the next 6 hours, then leave in the freezer until frozen hard.
Combine the strawberries, the remaining balsamic vinegar and some of the extra mint leaves in a bowl, and leave to marinate for 30 minutes at room temperature. When you are almost ready to serve, remove the sorbet from the freezer for 10 minutes to slightly soften.
Divide the strawberries with their juice between four serving glasses, and top each with a scoop of sorbet and a mint leaf.