Pick all the stalks from the blackcurrants, place the fruit in a saucepan, cover with 250ml/9fl oz water and bring to the boil. Simmer for 20 minutes, or until the skins of the currants are very tender and the liquid has almost evaporated.
Add the sugar and lemon juice, bring to the boil then cook until the mixture reaches 105C/220F on a sugar thermometer.
Leave to cool for a few minutes then pour into hot, clean jars and seal immediately.
