What is wrong with my Cox's orange pippin apple tree?

Can anyone help me identify what is wrong with my tree. It is approx 5-8yrs old. After a healthy leaf growth and a lot of blossom, the leaves have turned from healthy looking to discolored and sickly looking within a matter of a couple of days. I've attached some photos. The majority of the leaves are affected, but I can see very little in the way of insect infestation. Any help would be appreciated.