Place all of the ingredients into a lidded saucepan and bring to a simmer. Cover and simmer gently over a medium heat for 15 minutes.
Remove the lid and reduce the heat until the mixture is just simmering. Cook, stirring well to prevent the mixture from sticking to the bottom of the pan, for a further 30-40 minutes, or until the compôte mixture has thickened and resembles jam but is not too sticky. Set aside to cool slightly, then remove the plum stones.
When the compôte has cooled slightly, spoon it into two sterilised, lidded jars. Seal the jars, then set aside until completely cool. Chill in the fridge until needed. (NB: The compôte can be kept for up to two weeks).