Cut the stalks from the strawberries and cut the fruit into chunks. Tip into a large saucepan, add 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 completely dissolved.
Bring the mixture up to boiling point, then as soon as it starts to boil, leave to boil rapidly for 6 minutes. Either test for setting point using a sugar thermometer or spoon a little jam onto a chilled saucer. Chill for five minutes. Push your finger over the chilled jam and if the jam wrinkles up, it will have 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 skim off any froth from the surface of the jam. Leave the jam to cool for 15 minutes.
Meanwhile, wash the jam jars in hot soapy water and dry 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 eat. Once opened, store in the fridge.