Chop fruit finely or pulse in a food processor. Add to a large saucepan with the jam sugar.
Cook over a low heat stirring until sugar has dissolved. Add the knob of butter, increase heat and bring to a full boil (a boil that cannot be stopped when stirred).
Boil rapidly for 4 minutes only, carefully skimming off any foam from surface during cooking.
Remove from heat, then test a small amount on a cold plate for set, (the surface of the jam should wrinkle when a spoon is pushed through it). If not set, return to the boil for a further 2 minutes and test again.
Spoon into hot, sterilised, glass jars and seal. Makes 4 medium jars.