I saw it too on Alys program. I had a lots of plums in the freezer, which I had run out of ideas with, and have made and eaten lots of the stuff.
I did them at 60 in the oven, but I reduced them down in a preserving pan to 1/4 first then add the sugar if needed. I'm going to try the attic room next, it gets very hot up there and try the apple rings over the wood burner in the autumn.