Stone and quarter the plums, then put into a preserving pan. Add 500ml cold water and bring to a boil. Cover and simmer for about 45 mins until completely cooked down, pulpy and dark, dark red.
Sieve the fruit and juice through a nylon sieve back into the pan – make sure you get every bit of pulp out of the mix that you can.
Stir in the sugar, then stir over a low heat until dissolved. Now turn up the heat and bubble for about 25 mins or until you have a thick, dark and fruity purée. Keep stirring so that the bottom doesn’t catch – it’s ready when the spoon leaves a trail along the bottom of the pan for a split second before the paste floods back into the gap.
Pot the hot mix into small jars (using a funnel is easiest), seal, then leave to set. Will keep for up to 6 months.