Cut all of the plums into halves and remove the stones. Add everything to a large saucepan and mix in the white sugar. Cover with a lid and leave for few hours (or overnight, if you like)
Add the ginger and juice from one lemon.
Place the saucepan on a low to medium heat and gently stir the jam until the sugar has dissolved completely.
Once the sugar has dissolved, raise the heat slightly and bring the jam to a boil for around ten minutes, or until the jam starts to get thicker.
Take the saucepan off the heat and test the jam to see if reached its setting point.
The way to do this is to put a small teaspoon of the jam on a cold plate and if in about five minutes the jam firms up, its ready to be potted.
If the testing jam hasn't firmed up in 5 minutes on the plate, put the whole saucepan back onto the medium heat and continue boiling for another 5 minutes. Repeat the test and pot up when ready.