The same recipe can be used for other plums and gages but you may need to adjust the sugar to taste.
We don't put seals (greaseproof paper etc) over the top of the jam before putting the lids on because the jam won't be kept for that long (a year max, usually). But this is an option and may even be advisable if the jam's going to stay unopened for more than a year.
Here's the recipe:
Victoria Plum Jam (makes 10lb)
1/2 pint (275ml) of water
1/2 bottle Certo (pectinase)
1 Wash the plums, cut into pieces, removing any stones.
2 Put the fruit and water into a large pan - ideally a jam making pan - and heat.
3 Bring to the boil, stirring occassionally.
4 Simmer for 15 minutes.
5 While continuing to heat slowly, add the sugar and stir constantly until the sugar has dissolved.
6 Then bring to a full rolling boil and boil hard for two minutes, stirring occassionally.
7 Remove from the heat and add the Certo.
8 Skim if necessary to remove anything from the surface.
9 Pour quickly into the jars and put the lids on.