Wash an orange and pat dry. Thinly grate the zest, avoiding the bitter white pith.
Place the sugar in a small pan with 125ml water and bring to the boil. Stir in the orange zest and allow to bubble for 10 minutes.
Pour the orange syrup through a sieve and allow to cool in a freezerproof bowl set over iced water. Squeeze the juice from the grated orange.
Mix the campari and orange juice and pour into the orange syrup. Freeze for 45 minutes.
Stir the mixture well with a whisk. Freeze for another 21/2 hours, stirring occasionally.
Peel the remaining orange and remove the white pith. Separate the orange segments from centre.
Scoop out balls of sorbet and arrange on the orange segments. Decorate with herbs.