Weigh the grapefruit juice, the freshly squeezed grapefruit juice and Elderflower Syrup into a jug. Mix the three ingredients together.
Weigh the tonic water directly into the carafe and stir gently so as not to lose the fizz. Lastly, add slices of pink grapefruit and fresh mint leaves to the jug.
Serve the cocktail in a glass filled with ice.