1. Place the plums in a pan with the sugar and water. Stir over a gentle heat until the sugar is melted, then cover and simmer gently for about 5 minutes, until the fruit is soft.
2. Turn into a food processor and puree until smooth, then stir in the port. Cool completely, then tip into a freezer container and freeze until firm around the edges.
3. Spoon into the food processor and process until smooth. Return to the freezer and freeze until solid.
4. Allow to soften slightly at room temperature for 15-20 minutes before serving in scoops, with sweet biscuits.