Heat together the sugar, cinnamon, vanilla pod and 700ml of water. Bring to the boil once the sugar has dissolved, then simmer for 6–8 minutes. Remove the pan from the heat, stir in the port, orange juice and rind and pour over the prunes. Cover and leave in a cool place for 24 hours.
The next day strain off the liquid into a pan, bring to the boil and keep over a high heat until