Bring the grapes and water to the boil in a pan, reduce the heat. Cover and simmer until the grapes burst, leave to stand on the switched-off hob for approx. 10 mins., pass through a sieve (yields approx. 1 l juice). Pour the grape juice back into the pan.
Mix the lemon juice and half of the preserving sugar with the grape juice, bring to the boil while stirring, boil fast for approx. 1 min. Add the remainder of the preserving sugar, cook over a medium heat for approx. 4 mins., stirring constantly. Skim off any foam. Ladle the jelly into the clean, pre-warmed jars while still boiling hot, fill the jars to just below the rim, seal immediately. Briefly upend any jars with a screw-top lid, leave to cool on an insulating surface.