Place all the ingredients in a saucepan and simmer over a medium heat for about 20–25 minutes. As it simmers, some of the liquid will evaporate and the mixture will thicken. Once the liquid has reduced and the desired consistency has been reached, remove the saucepan from the heat.
Immediately transfer the hot mixture to sterilised preserving jars, ensuring they are filled to the brim to minimise air pockets. Seal the jars tightly and leave them to cool at room temperature. Once the jars have cooled completely, store them in the fridge. When sealed, the contents will keep for up to 6 months.
