Put the strawberries and lemon juice in a preserving pan.
Heat gently for 10 minutes, stirring all the time, to reduce the volume.
Add the sugar, stir over low heat until dissolved, then bring to the boil and boil rapidly until setting point is reached.
Remove from the heat and skim.
Leave the jam undisturbed to cool for about 20 minutes until a skin forms on the surface and the fruit sinks.
Stir gently to distribute the strawberries.
Pot and cover with discs.
Tie down and label when cold.