Put the sugar and juice into a jam pan or large heavy based saucepan. Bring to a simmer and allow to bubble until the sugar has dissolved.
Add the cranberries to the pan and stir well. Return to the boil and cook for 10 minutes so that the cranberries burst and the sauce thickens and becomes jammy.
Allow to cool a little and, if desired, use a stick blender to whizz to a smooth consistency.
Transfer to sterilised jars and keep in the fridge for up to 2 weeks.