Strip the currants from the stalks, then add to a preserving pan. Break up the fruit with a wooden spoon or potato masher, then add the measurement water. Cover and cook gently for 20–30 minutes, until the fruit is soft.
Pour the sugar into the pan and heat gently, stirring from time to time, until dissolved. Bring to the boil, then boil rapidly, stirring from time to time, until setting point is reached (about 20 minutes). Skim with a draining spoon or stir in butter if needed.
Ladle into warm, dry jars, filling to the very top, then 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 perfect for spooning over individual pavlovas topped with whipped cream.