Rinse the plums and pat dry before cutting in half but leave the stone in them as they add flavour and pectin.
Place plums in a pan along with the other ingredients and just a little bit of water so they don't stick to the bottom of the pot.
Cook until the plums are completely tender and squishy.
Remove from heat and press the pulp through a sieve with the back of a wooden spoon to get as much juice as possible.
Weigh the resulting juice and measure out exactly the same quantity of sugar.
Pour juice and sugar into a heavy based pan and slowly bring to the boil allowing the sugar to dissolve completely.
Lower the heat and simmer for approximately 40 minutes until it is thickened and has reached a good set.
Carefully ladle into a small jar and pop on the lid.