Any ideas?

When we moved in this tree had self seeded and survived the first year in our garden, despite frequent bonfires. Another popped up next to it the following year. They have quickly grown to a reasonable size and today I noticed that this year they have produced fruit. To me the trees looked like plum trees and this fruit does look plum like to me. So now it's a case of wait and see. Will we have some sort of edible yumminess?