Combine all basil sugar syrup ingredients in a pot over a high heat and bring to a boil.

Stir continuously until the sugar dissolves.

Reduce heat to low and simmer for 5 minutes.

Let sit for approximately one hour. Drain through a sieve and store in an air-tight glass container in the fridge. This can be kept in the fridge for 1 week.

Blitz watermelon in a blender or thermomix. Pour into a jug.

Top the jug with mineral water and add the basil sugar syrup (to taste).

Add ice and garnish with watermelon slices and extra basil leaves.