Add lots of ice to a large cocktail or wine glass and stir to chill the glass. Drain any melted water.
Quarter the strawberries. In a cocktail shaker, muddle the basil leaves. Add the strawberries and sugar and muddle again. Add the gin and a handful of ice. Shake until cold, and then strain into cocktail glass.
Add the garnishes and then pour the tonic water onto a bar spoon into the glass (to increase the bubbles). Stir once. Garnish with strawberries and additional basil leaves (important for the basil nose on the drink!) and serve.