If you don’t have a cooking thermometer, put a saucer in the freezer. Sterilise the jars you want to use. Tip the blackcurrants into a heavy-based saucepan with about 100ml of water. Bring to the boil and simmer for 5 mins until the fruit has broken down to a chunky pulp. Leave to cool slightly.
You now have two options. For a smooth jelly-style jam, squash the fruit through a sieve into a bowl. If you prefer your jam chunky and seeded, leave the pulp as it is. Whether it's strained or unstrained, weigh the fruit pulp and then add 400g of sugar to every 500g of pulp, then tip back in the saucepan.
Pour in the lemon juice then heat gently, stirring, to dissolve the sugar. Turn up the heat, then boil hard for about 10 mins or until it reaches 105C (setting point) on a cooking thermometer. If you don’t own a thermometer, test for setting point by spooning a little jam onto the cold saucer. After a couple of minutes gently push your finger through the jam – if the surface wrinkles, it's ready. If not, return to the boil for 2 mins, then re-test.
Take off the heat and skim off any froth with a slotted spoon. Cool for 10-15 minutes. Stir gently to distribute the fruit, then ladle into sterilised jars. Keeps for 6 months in a cool dry cupboard.