Put all the ingredients including the basil leaves into your shaker. Add ice and shake until very cool. Then strain into a tumbler filled with ice. Make sure you use the juice of freshly squeezed grapefruits to ensure the authentic flavour of the drink. With this cocktail do not cut down on garnish. Decorate it with lemon zest, basil leaves and a grapefruit slice to give it its paradisiac look. It will differ from the colourful, extravagant Caribbean cocktails only in its exquisite taste.