Cut the plums into quarters, removing the stone. Depitt the cherries beofre cooking them if you do not have a food mill.

Place all the ingredients in a pot and simmer them gently for 10 -12 minutes, until the cherries are soft.

Let the mixture cool, then pass it through a food-mill, or, if all the stones and pitts are removed, blend it.

Follow the instructions on your ice-cream maker to make the sorbet.  If you do not have an ice-cream maker, pour the cooled mixture into a high-sided tray and place it in your freezer.  Rake a fork through it every half hour until it becomes solid.