Add sugar, basil and 100g or 3.5 oz water to mixer bowl. Cook for 10 min/100°C or 212°F/speed 1/MC off. Pick out or strain basil off and allow syrup to cool for 5 mins.
Add watermelon, lime juice and ice to mixer bowl with the basil syrup. Blend for 1 min/speed 9/mc on.
Add gin and soda water and blend for 10 sec/speed 3/mc on.