The plums are not incredibly ripe, but they are soft and this is a good time to pick them for jam. I had some help from Ellie who picked a large bowl full. I washed them and removed any stalks, leaves etc, then chucked them in a large pan and heated gentle, without adding any water.
I had, before I started with the jam, washed and removed all the labels from the jars (I used pesto jars because they are quite small) and put them into the oven, wet, at 140oC to sterilise them. When they had dried (about 30 minutes) I turned off the heat and left them in the oven until I needed them, this meant the glass was still warm and so wouldn't crack when I added the hot jam
I didn't add the sugar with the fruit at the begining because apparently sugar has a hardening effect on the fruit, so fruit like plums which have tough skins should be simmered and softened first, but soft fruit like strawberries and blackberries are better if soaked in sugar first, helping to keep the fruit whole.
With lots of different advice, I decided not to stone the plums first, next time I will because I felt I had lost a lot of the flesh trying to remove the stones and ended up sieving the mixture to remove them all. It took about 15 minutes before the fruit started to soften enough to turn up the heat, I cooked the plums for about 45 minutes before I added the sugar.
I heated slowly until the sugar had dissolved and then turned up the heat to boil, boiled for about 10 minutes at "jam" temperature according to the themometer.
Then I poured the jam into the jars, ladled some first but this was just messy so I poured straight from the saucepan, filled to the top, added the lids and wiped the jars down, then made sure the lids were tight using a tea towel, because the jars were HOT!
Voila,