Begin this the night before you actually want to make the jam.Take a large casserole or small preserving pan and grease the base with a smear of the butter to prevent the preserve sticking. Halve the apricots and place them in layers in the pan, sprinkling the sugar in between the layers. Add the lemon juice, then cover with a clean cloth and leave them overnight – this pre-soaking in sugar firms up the fruit and this will ensure that the apricot pieces stay intact when you come to make the jam.To make the preserve, first pop three small plates into the freezer (this is for testing the set), then place the pan over a medium heat and let the sugar melt and completely dissolve – about 15 minutes. The sugar must be absolutely clear and free of granules, otherwise the preserve will be sugary. When the sugar has dissolved turn up the heat to its very highest and let the mixture boil very rapidly for about 10-20 minutes, stirring from time to time to prevent sticking. After that use the cold plates to test for a set. Remove the pan from the heat and place a teaspoonful of the preserve on one of the plates.Allow it to cool for a few seconds, then push it with your finger: if a crinkly skin has formed on the jam, then it has set. If it hasn't set, boil it again for another 5 minutes and do another test. When you have a set, remove the preserve from the heat and stir in a trace of butter, which will disperse any scum that has formed and let it settle for 15 minutes before pouring it into the warmed sterilised jars.Seal while still warm and label the jars when cold.