Wash the plums and pit them. Then place in a big pot together with all the other ingredients and 125ml water.
Bring the mixture to boil. Once boiling, turn down the heat and simmer for about an hour. If it sticks to the pot, add a bit of water.
Blend the marmalade until completely smooth, pour into clean jars and close. You can freeze the jars to keep the marmalade fresh for longer.