I tested the flavour essence tonight on a bottle of porter I bought, just to see what sort of flavour it gave. A flippin horrible one was the answer, not at all plum flavoured, tasted more like some sort of fruit chewing gum! Smelt like the kind of stuff people vape with.... So that isn't happening.
Guess I might actually have to try jam too, and just use pecolase. Just as well I have a bottle really. The recipe I have that uses damson has it getting boiled in a bit of water, and put in secondary though, otherwise the flavour just gets blown out of the airlock during primary.
Once again, my plum porter brew day will have to go on hold whilst I work it out....