Greengage jam is pretty hard to find in the shops, and plum jam hardly less so. This is a real shame, because they are both delicious, very easy to make and a great use for a glut of fruit.
Put the greengage stones into a jelly bag. Roughly chop the flesh (you do not want any large pieces of skin to persist in the final jam) and put into a large pan.
Add the lemon zest to the fruit. Put the squeezed lemon halves into the jelly bag, tie it up and put it in the pan.
Add cold water to the lemon juice to bring it to a total volume of 200ml/7fl oz