As far as I know ivy growing up a tree doesn't remove nutrients directly from the tree. Does the ivy block out any sunshine?

Hi, welcome to GoY, ivy growing up a tree doesn't usually cause any problems unless the tree is very old, or has been stressed in some way, and the weight of the ivy is causing more stress, it doesn't usually get into the crown of the tree, but if this is the case, there could be something causing your tree to become stressed, but it is very unlikely to be the ivy, Derek.

we had a frost shortly after the buds opened and as a consequence I had fewer apples last year. perhaps this happened to your tree.

Another possibility is that your tree has exhausted itself after bearing a heavy crop for several years. The danger is that it will now get into a two-year cycle - one year a good crop and the next a poor one - which is difficult to break. If there is a heavy fruit set this year, I would be inclined to remove about three quarters of the small fruit to try and encourage it into even cropping each year.

Just a thought.......Have you ever fed it?
I usually give a balanced fertiliser in the spring, last winter was very wet in some areas and the nutrients could have been washed away