Bring 150 ml of water and 150g of caster sugar to the boil in a saucepan stirring, until the sugar has dissolved.
Remove from the heat and whisk in the chocolate until melted.
Leave it to cool slightly.
Transfer the mix to a blender along with the clementine zest and 600 ml of water.
Blitz until smooth.
Pour the mixture into a container, cover and leave in the freezer.
Use a fork to break up the mixture every 30 minutes until set.
Once it has set, blitz the sorbet mix in a blender until smooth and then freeze until ready to serve.