Wash the blackberries and put them into a pan with the water. Bring to the boil and simmer gently until the juice runs from the fruit. Strain and leave the juice to drip through a jelly bag. Allow llb sugar to each pint of juice. Heat gently together until the sugar dissolves, and then bring to the boil and add the lemon juice. Boil rapidly for twenty minutes until setting point is reached. Put into warm pots and cover.