Place the sugar and water in a heavy based saucepan and slowly bring to the boil. Boil gently for 10 minutes.
Add the damsons and cook until soft. Strain, retaining the sugar syrup. Rub the damsons through a sieve to give a thick purée. Add this purée to the syrup and cool.
Turn the mixture into a shallow tin and freeze until fairly solid. Whisk the egg whites until stiff and gr