Tip the sugar into the boiling water in a heavy-based saucepan and stir until dissolved.
Cool and chill the syrup in the refrigerator.
Put the mango flesh and lime zest and juice in a food processor or blender, and blend until smooth.
Pour the chilled syrup into the food processor and blend for a further 30 seconds.
Transfer the mixture to a freezer-proof container and freeze for 1 1/2 hours.
Remove from the freezer and whisk to remove any ice crystals.
Return the container to the freezer and freeze the mixture until solid, whisking it every hour to prevent ice crystals from forming.