I have been making large batches of strawberry jam for years and always had problems with it setting. I now believe the secret to strawberry jam is to make it in small quantities.
2 punnets (500g) strawberries
1 vanilla bean, split and seeds scraped
3 tbsp lemon juice
1½ cups sugar
Hull strawberries and cut in half. Put in a heavy-based saucepan with vanilla bean and lemon juice and cook gently for a few minutes until the fruit is warmed through and softened.
Add sugar and stir to dissolve. When sugar has dissolved, increase heat and boil rapidly for 10-15 minutes or until setting point is reached. Stir jam to prevent it from sticking to the base of the pan.
Check for setting point by placing a teaspoon of jam on a cold saucer and placing it in the freezer for two minutes. The jam is ready if a skin forms when you push it gently with your finger and the jam separates into two parts when you run your finger through it. If the jam has not set, return to the heat and check again after a few minutes.
When setting point is reached, remove jam from heat, remove vanilla pod and carefully pour jam into hot, sterilised jars. Refrigerate jam when opened.
Tip: If the jam does not set, it may be the strawberries were too ripe. Homemade strawberry jam still tastes delicious when it's quite runny but if you prefer it firm, use a jam setting agent from the supermarket — Jamsetta is very good to thicken unset jam. Don't overcook jam as it will affect the flavour.