Stone and cut the majority your plums in half. Chop the rest into smaller pieces.
Put the plums in a preserving pan and add 200ml water. Bring to a simmer and place on a lid.
Cook for about 10 - 15 mins until the plums are tender but not falling apart.
Add the sugar and, over a low heat, let the sugar dissolve slowly without boiling. This is likely to take about 5 mins.
Check the sugar has completely dissolved before increasing the heat to medium.
Bring to a gentle rolling boil and cook for around 15 minutes or until the jam thermometer reaches 104℃ / 220F.
Test for setting point using the wrinkle test. Remove your pan from the heat and drop a little jam onto an ice cold saucer. Then gently push the jam with your fingertip - if the jam crinkles, setting point has been reached. If not, boil the jam for a couple of minutes more and then check setting point again.
Once setting point is reached, ladle your jam into warm, sterilised jars before sealing with airtight lids.
Store in a cool, dry place out of the sunlight and use within 12 months.