Wash the grapes and pick off the bunch, carefully removing the stalks. Add the fruit to the preserving pan with the lemon rind and juice and the measurement water. Cover and simmer gently for 10 minutes, until the juices have run and the fruit has softened.
Roughly mash the grapes with a potato masher, then add the warmed sugar and cook gently, stirring from time to time, until the sugar has completely dissolved. Bring to the boil and cook until setting point is reached (10–20 minutes). Skim with a draining spoon or stir in butter if needed.
Leave to cool for 10 minutes so that the grapes don't rise in the jars, then 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 conserve tastes good with rustic bread and butter.