Tried it again with Damson plum jam from the same farm, but pulled back the lemon juice to 20ml and increased the sugar syrup. My lemons were VERY tart, so with the Damson plum, it was still a bit much, as Avery notes below.. On a whim I added a bar spoon of grenadine and it actually blended nicely with the lemon, plum, and apple flavors. You probably have to enjoy grenadine to do this.
I used Damson plum jam bought from a local orchard. I agree that it was a bit more bitter than I like, but think it was more about balancing the lemon juice and simple syrup. The jam with apple brandy is fantastic.
Also, the Damson plum jam may not dissolve evenly, so I recommend stirring it in with the apple brandy or calvados and then shaking, and then using a fine strainer to catch the undiluted jam (unless you like your cocktail with globules of jam).
I did actually have a homemade damson jam and this was delicious but it /definitely/ had too much lemon. Maybe the original recipe uses a very sweet damson jam?