Combine lemon rind and juice, grapes, blackberries, blueberries and lemon grass in a large heavy based saucepan. Bring to the boil and cook for 2 mins.
Add sugar stirring over a medium heat for 1 min or until sugar dissolves. Bring to the boil and boil rapidly for 4 mins (removing any foam with a tablespoon).
Test a small amount of jam on a cold saucer (the surface should wrinkle when a spoon is pushed through it).
If surface doesn’t wrinkle when testing for ‘set’, return to boil for 2 minutes and repeat the test.
Ladle hot jam into warm sterilised jars and seal with a lid. Set aside to cool, then label, date and store in a cool dark place. Refrigerate after opening.
Recipe as seen on the Chelsea Sugar website