Wash jam jars and place upside down in the oven at 100C to dry thoroughly.
Place a small plate in the fridge for testing the jam later.
Hull the strawberries and add half to a preserving pan and crush them roughly with a potato masher; add the rest of the fruit, the sugar and the lemon juice.
Stir all the ingredients together, well off the heat.
Place the pan on a low heat and stir until sugar has dissolved.
Add the butter.
Bring to a rapid, rolling boil for 4 mins.
When cool, spoon a little on to a chilled saucer, run a finger through the jam so that the saucer is visible.
If the finger line remains in the jam and the jam wrinkles it is ready. If not boil a little longer and try again.
Ladle into the hot jars and fill until almost full to allow for shrinkage as the jam cools. Cover the surface of each jar with a waxed disc, with the shiny side down.
Rinse a cellophane circle with a little water then put wetted side uppermost over the top of the jar and secure with an elastic band or a screw-top lid.
Recipe by appetite magazine at https://www.appetitemag.co.uk/strawberry-jam/