Add the fruit to a preserving pan with the measurement water. Cover and cook gently for 20 minutes, stirring from time to time, until the fruit is just beginning to soften.
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 (20–25 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 can be used as the filling between 2 shortbread biscuits, the top layer having a small heart shape cut out before baking.