Begin by washing your jars and lids in hot soapy water, then put the jars in the oven at 100°C / 210 °F / Gas ½ to sterilise.

Wash, stone and quarter the plums, then put them in a big pot with the orange juice and water. Bring to a simmer and cook really gently, stirring often to prevent the skins sticking to the bottom of the pot until the fruit and skins are soft. This will take 15 to 20 minutes, but don't rush it. It is important that the skins are softened before adding the sugar as otherwise they will harden.

Add the sugar and cinnamon and stir until it is completely dissolved, then turn the heat up and bring to the boil until setting point has reached (104.5°C or do the 'wrinkle test' with a plate from the freezer - see note). Usually I would say not to stir the jam during this time but plum jam has an unfortunate habit of sticking to the bottom of the pan, so keep an eye on it and stir occasionally.

When the setting point has been reached,remove the jam from the heat, stir in the walnuts, orange zest and rum or brandy (if using).

Pour into sterilised jars (a jam funnel will make life easier here) and seal.

Store in a cool, dark place for up to 12 months and refrigerate once opened.