Add half the strawberries to a preserving pan and crush them roughly with a potato masher. Add the remaining strawberries and the lemon juice and heat gently for 15 minutes, until the fruit has softened.
Pour the sugar into a preserving pan and heat gently, stirring from time to time, until dissolved. Bring to the boil, then boil rapidly until setting point is reached (5–15 minutes). Skim with a draining spoon or stir in butter, if needed.
Wait for 5 minutes before ladling the fruit into the jars to prevent the fruit from rising in the jar. Ladle into warm, dry jars, filling to the very top, then cover with screw-top lids, or with waxed discs and cellophane tops secured with elastic bands. Label and leave to cool.