Combine sugar and 2 cups (500ml) of water in a medium saucepan. Stir over low heat, without boiling, until the sugar dissolves. Bring to the boil. Add the mint sprigs, reduce the heat and simmer, without stirring, for 5 minutes.

Meanwhile, blend or process the watermelon in batches until smooth. Push puree through a sieve into a large bowl. Stir in the syrup.

Pour the mixture into two shallow meta pans. Cover with foil and freeze for about 3 hours or until almost set.

Using a fork, scrape the granita from bottom and sides of the pans, mixing the frozen with the unfrozen mixture. Cover and return to the freezer until frozen.

To serve, use a fork to scrape the granita to form large ice crystals. Spoon into glasses and serve immediately.