Place all ingredients in a large, heavy pan and bring to a gentle simmer, stirring frequently.
Simmer gently for 60 minutes, bring to a gentle boil. Continue to cook until the mixture turns dark and thick, with a shiny glaze. Place into sterilised jars leaving 1/2 headspace, allow to cool a bit before sealing and processing in a hot water bath or pressure canner.
Store in a cool, dark cupboard for up to one year.