In true blog style I should start off by saying that I had a glut of plums but I didn't. I bought these from the supermarket because they were cheap and they were more like bullets than plums. They would have made your gums bleed, they were that hard.
I confess this is not my recipe but one I found on a yahoo forum thread.
LOW SUGAR PLUM JAM
5-6 cups of plums cut up, stones removed
1 cup sugar
juice of 1 lemon
2 teaspoons of ground cinnamon
Combine the cut up plums and sugar in a large pan. Leave overnight or for 24 hours. After this add the lemon juice and ground cinammon. Bring to the boil, then simmer until setting point is reached. Depending on how much fruit you have and how ripe it is, this could take 30 mins to 1 hr.
I made 4 batches of these all in one go, that is 20 cups of plums. Just calculate the other ingredients accordingly.
If 1 cup of sugar sounds like a lot to you, normally a plum jam recipe would require almost the same amount of sugar to fruit. 1 cup of sugar is a lot less than 5 cups!
If you bottle this in sterilised jars, this will keep for up to 2 years.
To test for setting point, have a china saucer in the fridge. Drop a dollop of jam onto it, put back in the fridge. After 1-2 minutes, run your finger through the jam. If it wrinkles, you have setting point.
To bottle the jam, wash the jars and caps very well and rinse. If you have a dishwasher, use this. Then put them in the oven, set temperature to 100C/ 212F/ gas 3 until they're hot and dry. Pour jam in using a funnel or jug. Using gloves or a tea towel, close the jar and turn upside down to ensure a good seal, then back up again. If any jam leaks out, you haven't closed it properly.