Make a sugar rim on the glass by dipping the rim of the glass in a sauce of water. Spread the sugar on another saucer and dip the rim of the glass in the sugar.
Place the basil leaves and cucumber in a cocktail shaker and muddle (or gently bash) with a muddler or wooden spoon until the basil and cucumber are bruised and the scent is released. Add the sugar syrup, gin, lemon juice and ice. Shake vigorously until frost forms on the outside of the cocktail shaker.
Fill your glass with ice and strain the cocktail into the glass. Top up with tonic water. Decorate with strips of cucumber and basil leaves.