Place water, plums and apples into a preserving pan and simmer until fruit is softened (about 15 minutes).
Add sugar over a low heat and stir until dissolved (do not simmer).
Bring to a rolling boil and time for 12 minutes, stirring constantly.
Take off the heat and test on a chilled plate (place in fridge for 30 seconds), if it crinkles (is jelly like) when you drag your finger through it, it’s ready.
If not boil for another couple of minutes at a time and repeat test.
Skim off any foam from the surface with a spoon.
Ladle the jam into the hot sterilised jars using a jam funnel if you have one (it makes it much easier) and screw the lids on immediately..
Will make 10 - 12 small to average sized jars, about 1.68 litres.