Put a small plate in the refrigerator to chill before you begin. Halve and pit the apricots. Crack a few of the pits with a hammer and take out the kernels; discard the rest.
Put the kernels into a heatproof bowl and pour in enough boiling water to cover. Leave for 1 minute, then transfer to a bowl of cold water. Drain again and rub off the skins with your fingers.
Put apricots, kernels, lemon juice, and 10fl oz (300ml) water into large stainless-steel pan, bring to a boil. Lower heat and simmer, stir occasionally, for 25 min, until apricot skins are soft, fruit tender, and mixture reduces by one-third.
Add the sugar to the pan and stir until it completely dissolves. Increase the heat and bring the mixture to a boil, without stirring, for 10 minutes, or until it reaches the setting point. Remove the pan from the heat and test for a set.
With the pan still off the heat, use a large metal spoon to skim the surface and remove the kernels. Immediately pour the jam into the prepared jars to within ... in (3mm) of the tops and seal. Let the jam cool, then label and date the jars.
Store in a cool, dark place until ready to use, then refrigerate after opening.