Just two jars...

Boiled up the damsons and squashed them through a sieve. Got the pips out, then put the puree in a saucepan with the same weight in sugar (now THAT made me think!). The instructions said to boil it up and then to reduce the heat and cook until 'very thick'.  After about an hour of this gentle cooking I decided the thick stage was just not happening for me! So I walloped the heat up full blast and - hooray - within ten more minutes I could just about stand the spoon up in it.

The mixture filled not quite two jars. A lot of effort for not much. I'd best pick more next year, assuming it tastes as good as it looks.

That is, if the mixture has not set like toffee in the jars.........