Place the sugar, Mint and one third of the water in a saucepan. Heat gently until the sugar has dissolved. Bring to the boil. Remove from the heat and leave the mixture to stand for 5 minutes. Strain the liquid through a fine sieve, discarding the Mint. Stir in the remaining water, lemon juice, orange rind and juice and the rose-hip syrup. Mix well.
Pour the liquid into a shallow dish and place it in the freezer compartment of the refrigerator for about 2 hours, or until the mixture is half frozen. Beat the egg white until it is stiff. Add the caster sugar to the egg white and whisk it again until it is thick and shiny.
Take the half-frozen mixture and whisk it until it is almost liquid, but with the icy fragments still visible. Fold in the beaten egg white and sugar. Freeze the sorbet for 2 hours, then re-whisk it. Place the sorbet in the freezer compartment for a further 2 hours, or until it is frozen.
Serve in individual glasses and garnish with wafers or biscuits. The sorbet may be stored in the freezer for 3 months.