Bring the sugar and water to the boil in a small pan, simmer until the sugar has dissolved, leave to cool.
Fill the glasses with ice cubes. Place ¼ of the basil in the jam jar, crush gently with a ladle. Add 3 tbsp of gin, 2 tbsp of lemon juice and 2 tbsp of sugar syrup, fill the jar with ice cubes, seal tightly. Shake the jar well for approx. 30 secs. Pour the cocktail through a sieve and into a jar, garnish with the reserved basil leaves. Repeat this step three more times.