Remove the stones from the fruit then put the damsons in a deep pan with 300ml water and simmer for 10-15 minutes until soft.
Stir in the jam sugar and cook over a low heat, stirring frequently, until the sugar has dissolved. Bring to a rapid boil and boil for about 15 minutes, then take off the heat.
To test for the setting point, spoon a little jam onto a chilled saucer, then freeze for 2 minutes. Push your finger through it. If it wrinkles and isn’t runny, it’s ready. If it doesn’t wrinkle, return to the heat and boil for 2-3 minutes more, then re-test. Repeat the test until ready.
Take off the heat and cool for 15 minutes.
Stir the jam so the fruit is suspended in the syrup, then ladle into warm sterilised jars. Cover immediately with waxed discs and lids, label and date. The jam will keep in a cool dry place for months.