450g jam sugar

Juice 1 lemon

Cook the grapes over a low heat for 5 minutes until the juices start to run. Bash the grapes brutally. Leave to cook for a further 10 minutes with periodic savaging with a potato masher. But the grapes into a clean cloth and leave to drip through over-night. 

Measure out the juice (you should get about 600ml). Poor into the pan with the sugar and lemon juice. Bring to the boil until it reaches 105C (seriously just get a thermometer it makes your life much easier). Do the usual jam things to pot up and check if it is set.