If you don't have a cooking thermometer, freeze a saucer. Prepare the jars for use by sterilizing them. Place the blackcurrants in a heavy-bottomed pot with 100ml of water. Bring to a boil and continue to cook for 5 minutes, or until the fruit has broken down into a chunky pulp. Let to cool slightly.

You now have two alternatives. Squeeze the fruit through a sieve into a basin for a smooth jelly-style jam. If you want your jam chunky and seedy, let the pulp alone. Weigh the fruit pulp, whether strained or unstrained, and then add 400g of sugar for every 500g of pulp, then return to the pot.

Throw in the lemon juice and slowly heat, stirring constantly, to dissolve the sugar. Turn raise the heat and boil for about 10 minutes, or until it reaches 105 degrees Celsius (setting point) on a cooking thermometer. If you don't have a thermometer, test the setting point by spooning a little amount of jam onto a cool saucer. After a few minutes, carefully poke your finger through the jam to see whether the surface creases. If not, return to the boil for 2 minutes before re-testing.

Remove from the heat and, using a slotted spoon, skim off any froth. Let to cool for 10-15 minutes. Gently stir in the fruit, then ladle into sterilized jars. In a cold, dry place, it will keep for 6 months.