In a preserving pan, combine the strawberries with half of the sugar and boil for 20 minutes. Remove from the heat and cool.
Return to the heat with the remainder of the sugar and bring to the boil. Begin testing for the setting point (this could take up to 30 minutes) by dropping a small amount of jam onto a cold plate. If the jam wrinkles when you push it with your finger, it is ready.
Transfer to sterilised jam jars and seal immediately. Leave undisturbed for 8 hours or overnight to achieve a good set.