I found one that also calls for peaches. You could just use all plums if you want to.

PEACH - PLUM FREEZER JAM

3/4 lb. fresh plums
1 lb. fresh peaches, peeled & chopped
2 tbsp. lemon juice
1/2 tsp. ascorbic-citric powder
4 c. sugar
3/4 c. water
1 (1 3/4 oz.) pkg. powdered pectin

Remove pits from plums (do not peel) and grind pulp finely. Measure 1 1/4 cups peaches and 1 cup ground plums. Combine fruit, lemon juice and ascorbic-citric powder in a large bowl. Add sugar to fruit, mixing well. Let stand 10 minutes. Combine water and pectin in a small saucepan. Bring to a boil and boil 1 minute, stirring constantly. Pour over fruit mixture and stir 3 minutes. Quickly pour into freezer containers or hot sterilized jars, leaving 1/2 inch headspace. Cover at once with plastic or metal lids and screw on bands, if appropriate. Let stand at room temperature 24 hours; then store in freezer. To serve, thaw jam. Yields about 6 half pi

Honey Plum Jam
3 cups pitted, coarsely chopped purple plums (about 500g)
1/3 cup sugar
1/3 cup water
2 tablespoons lemon juice
2 tablespoons honey

Combine the plums, sugar, water and lemon juice in saucepan. Bring to the boil, reduce heat, but maintain boil. Stir and mash occasionally with back of spoon for about 15 minutes, or until thickened. Remove from heat, stir in honey. Allow to cool to room temperature. Refrigerate in tightly covered jars for up to 2 weeks.

Plum Jam
You will need 340g sugar for every 450g plums. Wash and dry, stone and weight the plums and cut into halves or quarters. Put into a large saucepan, cover with the sugar. Stir well and leave for 24 hours. Next day, slowly bring the mixture to the boil, stirring with a wooden spoon at short intervals until the sugar starts to dissolve. When boiling point is reached, reduce heat and simmer on low until the mixture gels when tested. Stir in 2 teaspoons margarine. Bottle in sterilised jars while still hot and seal with paraffin wax. If liked, add 1 tablespoon chopped glace ginger and 1 tablespoon green ginger wine for every 500g of jam to the mixture when beginning to cook.

Hope this is not too late, but there is another plum honey jam recipe that calls for just plums, and honey. It is very good with blue or yellow plums, and one time I mixed in a few peaches as well. Good good!

Here is a link that might be useful: Plum Honey Jam