Add the water and castor sugar to a saucepan and stir on medium heat until the sugar is dissolved. Then add the rosemary and mandarin rind. Boil the mixture for 5 minutes. The liquid will reduce slightly and thicken a little. 

Turn off the heat and add in the mandarin juice. Stir though and let this mixture cool completely. Then strain the mixture to catch the fresh rosemary and mandarin pulp. Discard this and then place the liquid in the fridge for at least 2 hours to completely cool down. 

Then add this mixture to an ice cream mixer and follow the instructions, or freeze into ice trays overnight and then blend in a food processor the next day and put the mixture back in the freezer or eat straight away!