Peter
I have noticed a similar condition on my Charles Ross trees. I have
two thoughts
firstly as Gill suggests - an early June drop, but those falling are
very imature.
Second thought is the dry weather we had after blossom set. We had
close to 8 weeks with no rain worth mentioning, so I wondered if the
trees were just responding to conditions. Despite the rain, there are
patches in my garden where the sandy soil is still dry.
Have a look at the apples that have fallen - cut them open and see
what they are like with a hand lense. Any sign of insect damage,
particully around the blossom end of the fruit? If so the tree will
reject, and drop the apple.
Regards
Clifford
Bawtry, Doncaster, UK