Wash the redcurrants and put them, stalks and all, into a large preserving pan along with 500ml/18fl oz water and the mint sprig. Slowly bring to the boil.
Stir and press the redcurrants to break down the fruit.
Remove the pan from the heat and cool slightly. Tie a jelly bag (or a clean sheet of muslin) to something appropriate so it can dangle over a bowl. Carefully pour the contents into the jelly bag (or muslin). Cover and leave to drip overnight - do not stir or squeeze the jelly bag.
When all the juice has dripped through, transfer the juice to a preserving pan, along with the sugar and lemon juice.
Put a plate in the freezer.
Place the pan over a gentle heat and cook, stirring frequently, until the sugar has dissolved.
Bring to a rapid boil and boil for 10-15 minutes, or until setting point is reached. To test the setting point, remove the pan from the heat. Take your saucer from the freezer and place a drop of jelly onto the cold plate. After a few seconds push the jelly with your finger. If the jelly surface wrinkles then it has reached setting point and is ready. If it slides about as a liquid, then it hasn't reached setting point and should be returned to the heat and boiled for a few more minutes before testing again.
Remove from the heat and stir in the chopped mint.
Cool slightly before pouring into sterilised jars.
Cover with a lid and label.