Put the apricots in a large pan, add 1.5 litres water, cover and leave overnight to soak.
Next day, place a small plate in the fridge to chill. Put the apricots and water on the hob, then add the lemon juice and bring to the boil. Reduce the heat, then simmer for 30 mins or until the apricots are tender and starting to break up.
Remove from the heat and add the sugar, stirring until it dissolves. Return to the heat and boil rapidly for 20 mins or until setting point is reached. To check this, remove the plate from the fridge, put a spoonful of jam onto the plate and pop it back in the fridge for a few mins – the jam should wrinkle when you push it with your finger. A sugar thermometer will also give you the setting point of jam, which is 105C.
Carefully pour the jam into hot sterilised jars, top with a disc of wax paper or baking parchment, seal with a lid, then leave to cool and set. The jam will last for 6 months unopened in a cool, dark cupboard.