To make the sugar syrup
Place the sugar and water in a small saucepan over a medium heat. Keep stirring until the sugar dissolves, bring to a boil and remove from the heat.
Add the lime juice and the mint and set aside to cool.
Strain the liquid and decant into a small bottle or a jar and store in the fridge.
To make the watermelon & gin slushies
Place the frozen watermelon, frozen raspberries, gin and 2-3 tablespoons of sugar syrup in a blender and whizz until well combined.
Pour into glasses, garnish with fresh mint and enjoy!