Method Wash, drain and hull the fruit and put in a large non-metallic bowl. Sprinkle over the lemon juice and sugar, gently mix. Cover with a tea towel and leave overnight. (This helps keep the strawberries whole.) Put a saucer in the freezer. Tip the fruit and juice into a preserving pan or a 4.5 litre/8 pint heavybased pan. Heat gently, stirring, to dissolve the sugar. Do not boil until it has. Turn up the heat, then boil hard for exactly 4 minutes (use a timer). Take off the heat to test for setting point. Spoon a little jam onto the cold saucer. After a couple of minutes gently push your finger through the jam and if the surface wrinkles it is ready. If not, return to the boil for 2 minutes, then re-test. Take off the heat and swirl in the butter. If the scum doesn\'t dissolve, skim with a slotted spoon. Cool for 10-15 minutes. (Pot when too hot and all the fruit rises to the top.) Stir gently to distribute the fruit, then pour into warm sterilised jars. Put waxed discs on straight away, cover with lids or cellophane circles, then seal, label and wipe the jars. Keeps for 6 months in a cool dry cupboard.