Place the chopped plums, sugar and orange juice in a deep sided saucepan over a medium low heat. Stir occasionally until the sugar dissolves.
Once the sugar has dissolved, raise the heat slightly and cook over a medium temperature until the mixture boils, stirring and mashing the plums as they release their juices.
Continue boiling for about 10 minutes, then remove from the heat, add the cinnamon and stir through.
Notes
This recipe makes between 1 and 1½ cups of plum sauce dependent on the size of the plums and how long these have cooked down.