Cut the stalks from the strawberries and cut the fruit into chunks. Place in a large saucepan with the lemon juice and cook over a medium heat for a few minutes. Pour in the jam sugar and stir over a low heat until the sugar has completely dissolved.
Bring the fruit up to boiling point and then as soon as the jam starts to boil, leave to boil rapidly for 6 minutes. Test for the setting point: spoon a little jam onto a chilled saucer and chill for five minutes. Then, push your finger over the chilled jam – if the jam wrinkles up, it has reached the setting point. If not, boil again for a few minutes and re-test.
Remove the pan from the heat. Using a large spoon remove any of the froth from the surface of the jam, then allow the jam to cool for 15 minutes.
Meanwhile, wash the jam jars in hot soapy water and dry off in a hot oven to sterilise. Spoon the jam into the jars and top with a waxed paper disc. Seal and store until ready to enjoy. Once opened, store in the fridge.