Hull and wash the strawberries and drain well. Put into a large saucepan and mash or crush (a liquidiser may be used if available). Lemon juice may be added now, if liked.
Add the Sure-Set and heat stirring until it is dissolved, then bring to the boil and boil vigorously for 4 minutes. (See 'Adding Sure-Set' for details on boiling and timing).
Allow to cool for a few minutes, stir to distribute the fruit, then pot and cover in the usual way; label and date. (See 'Potting and covering' for details).