- Put the sugar and water into a small saucepan and bring to the boil. Simmer for 2 minutes to form a syrup, then set aside until cold.
- Cut the rind off the rockmelon and scoop out the seeds. The remaining flesh will weigh 250-300g. Put the flesh into a processor and pulse until smooth.
- Add the cold syrup and cream. Pulse briefly to combine. Pour the mixture into a plastic ice-cream container, cover and put into the freezer.
- When the sherbert is almost frozen, scoop the mixture back into the processor and pulse until very smooth, but not thawed.
- Put back into the ice-cream container, cover and freeze.