Q.plum fruit
Last year most of the fruit was inedible. There were white slugs inside, although they looked ok from the outside. Would a band around the tree help and when should it be applied?
Please take a look at the photo on this page - did the slug look like this?
http://nysipm.cornell.edu/factsheets/treefruit/pests/pc/pc_fig5.asp
If so, your tree was infested with the plum curculio beetle pest. The following article will give you more information and prescribe a course of treatment.
https://www.gardeningknowhow.com/edible/fruits/plum/plum-curculio.htm
If that wasn't it, let me know and I will go back to the drawing board!