Combine the sugar and water in a pan, bring to the boil. Simmer until the sugar has dissolved completely, leave to cool.
Pour 3 tbsp of sugar syrup, the gin, lemon juice and mixed berries into a shaker, top up with ice cubes. Seal the shaker and shake well for approx. 30 secs. Divide the ice cubes between the glasses, pour the cocktail into the glasses through a sieve, add the tonic water, decorate with berries.