Wash the strawberries and allow to dry, then hull. Place in a preserving pan or large saucepan and add the sugar.
Add the lemon juice and zest and stir well. Use the side of the spoon or a a potato masher to lightly crush some of the strawberries so that the juice starts to run.
Place over a low heat and heat gently while stirring frequently until all the sugar has dissolved.
Once all the sugar has been dissolved bring the mixture to a rolling boil and boil rapidly until setting point is reached (see below). This should take about 5 minutes.
Skim off any scum, then stir in a knob of butter and stir to disperse any remaining froth.
Allow to cool for about 15 minutes before potting into sterilised jars, cover and seal immediately. Label when cold.
Notes
Notes: To test for set – Chill a couple of saucers before you start. After about 5 minutes boiling turn off the heat. Place a small amount of jam on the chilled saucer and allow to cool for a minute or two. Gently push with your finger tip. If the surface crinkles then setting point is reached. If not return the mixture to the boil and boil again for a further minute or so then test again. Repeat until setting point is reached To sterilise jars – Wash your jars and the lids in hot soapy water. Rinse and place upside down on a roasting tray while they are still wet. Place in a preheated oven at 160-180ºC for about 10 mins.