Scab. A common problem with Apple trees. There are fungicides available if you do not mind spraying your food with chemicals. I used to use Copper sulphate, but they have stopped selling it over here.
If the tree is producing good clean fruit and is otherwise healthy you could ignore it, just make sure there is good airflow through the branches and clear up the fallen leaves.

I had a bad attack last year and my apple tree was almost totally defoliated. I spotted it too late and sprayed it too late. It's hard to spray an apple tree in Scotland as well, because of the wind. Hardly any still days. But I did buy some Copper sulphate as a powder which can be made into a spray solution. I got it for my potatoes to protect them from blight. The man in the GC advised me. They have stopped advertising it as a fungicide, but it's still out there. This year I did manage one spraying about a month ago and so far so good, but it really needs doing every 10 days and there's no way I can do that. I plan to hard prune my trees next winter in an attempt to re-invigorate them, having already thinned them out to increase air flow.

Thanks for the info Owdboggy and Cottagekarer. There is a good airflow throw the branches so I'll look for some copper sulphate to treat the tree. I live in Hull, East Yorkshire, England, and we too have some strong winds, but not like Scotland....thanks again.

I'm from near there, so yes, not dissimilar climate to me actually...but you get warmer days than us. Good luck with it!