Wash the grapes well and put them in a preserving pan or large stainless-steel saucepan with the lemon chunks and enough water to cover. Bring to the boil, skim, then simmer gently for 1 hour.
Strain, discarding the solids, and measure the juice. Return the juice to the pan with its own volume of sugar. Bring gently back to the boil, stirring with a wooden spoon till the sugar is comple