1. Combine the water, mint, and sugar in a saucepan. Place over a high heat and bring to the boil, stirring until the sugar is completely dissolved.
2. Reduce the heat and simmer, uncovered, for three minutes.
3. Add the grapefruit juice. Press the mixture through a fine-mesh sieve and discard any solids. Allow the mixture to cool completely.
4. Freeze mixture in a shallow non-metal container. Every half an hour, remove it from the freezer and stir with a fork, crushing any lumps, for about three hours or until evenly frozen.
5. Serve in glasses or bowls, topped with some fresh mint leaves.

Nutritional information:

Per serving: 89kcals, 0.1g fat (0g saturated), 23.2g carbs, 22.1g sugars, 0.6g protein, 1.1g fibre, 0.001g sodium