Place 1.4kg plums, half a cup of water and a tablespoon of lemon juice in a large non-aluminium saucepan.
Stirring often, bring to a boil over medium-high heat.
Cover, reduce the heat to low, and simmer 10 minutes, stirring occasionally, until the fruit is soft. You should end up with about 4 1/2 cups. Stir in 1.4kg sugar.
Return the heat to high and bring back to a rolling boil while constantly stirring. Add 85g Jamsetta and return to a boil while stirring.
Continue to stir and boil for 1 minute, then remove from heat. Let rest for 1 minute, then skim off any foam. Pour even amounts into sterilised jars, leaving 3mm of space at the top. Wipe rims and seal with sterilised lids.
Process in boiling water for five minutes.
Remove, let cool, and label jars. Store the jam in a cool, dry place. Makes about eight cups.
