Turn your back for a minute and look what happens:
I believe the culprit is brown rot, which apparently sets in after the fruit has been damaged by a bird or insect. So, the pictured apple has now been removed, though it looks like it could be joined by another.
Oh dear…. After all the trouble I took to keep the aphids off the tree. However, so far there are still 10 healthy apples which may prove to be ready to pick. So, I’ll assess the one I’ve taken off this evening for ripeness and take it from there.