The night before you need to pat the strawberries clean with a damp kitchen towel – do not wash or the jam may not set. Discard any bruised ones.
Layer the strawberries in a large heavy based pan with the preserving sugar and give a gentle stir before you go to bed. This helps firm up the strawberries.
The next day put 10 small strawberries aside and then using a potato masher crush the remaining strawberries gently.
Put the 10 strawberries back into the pan and gently heat the pan to slowly dissolve the sugar. Check with a spoon that all crystals have dissolved.
Now bring the pan to a rolling boil and add the juice of a lemon and ½ the skin (washed if waxed).
Time for exactly 8 minutes then take off the heat.
Put a teaspoon of jam on the plate and allow to cool completely.
Push your finger through the jam and if it forms crinkles, sits proud and doesn’t ooze liquid then it has reached setting point.
If not, boil for another 3 minutes and retest and so on until set.
Stir in the butter which will help to remove any foam (or you can remove with a slotted spoon).
Pot up the jam into the hot jars using either a sterilised jam funnel or I used a sterilised small fruit ladle which worked well holding the jars with a small oven proof square or a folded tea towel.
Put lids on immediately or use wax discs and cellophane lids with elastic bands and allow to cool before labelling.