Wash and halve the plums leaving the stones in. I like to crack some open and add some of the kernels found inside them here. The stones also contain pectin which helps the jam set. We'll skim the jam later so don't worry about leaving the skin on and stones in

Put the plums, kernels and water in a pan and simmer for around half an hour until the fruit is soft and liquid has reduced a bit

Add the sugar and keep stirring well to dissolve. Boil rapidly until the setting point is reached, skimming the top as required to remove the stones and skin

To check the setting point, pour a teaspoon of the jam onto a cold plate. If the jam wrinkles when you push it along with your finger, that's it ready. If not, keep boiling but check again in a few minutes

Have sterilised jars ready.Wash them in hot, soapy water then put them in the oven on the lowest setting for 15 minutes. Pour the jam while still hot into the warm jars. Fill to a half cm from the top. Put a circle of wax paper on top, then a dampened piece of cellophane held in place with an elastic band if you don't have a lid. Otherwise just the wax paper and lid.