Place the cranberries in a large, heavy-bottomed pan with the caster sugar, water, orange juice and zest, and the cinnamon stick and bring to a gentle simmer.
After ten minutes when the cranberries begin to soften and start to burst open carefully mash with the back of a fork to let out all the pectin.
Add in the port, stir and cook on a gentle heat for ten more minutes or until the sauce has reduced by a third.
Remove from the heat, and allow to cool a little. Remove the cinnamon stick.
Carefully ladle into warm, dry, sterilised jars and seal.
Notes
You can omit the port if you wish and replace with water.