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: 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.