Add the marrow, plums and blackberries to a preserving pan with the measurement water, then cover and cook for 20 minutes, until the fruits are just tender.
Pour the sugar into the pan and heat gently, stirring from time to time, until dissolved. Bring to the boil, then boil rapidly until setting point is reached (15–20 minutes). Skim with a draining spoon or stir in butter if needed.
Ladle into warm, dry jars, filling to the very top. Cover with screw-top lids, or with waxed discs and cellophane tops secured with elastic bands. Label and leave to cool.
To serve, this jam is delicious simply eaten with bread and butter.