what to plant under an apple tree?

I would wait to see if the apples are any good. If they are a good variety, come at the right time and taste good, and ideally can be stored for a time, then you should do as Marguerite suggests. We always marvel at the heavily cropping apple trees in the professional orchards over here in the heart of an apple growing area, but they keep the ground fairly clean and use trickle irrigation, including nutrients in the water. And the trees are all under a netted area to protect them from birds and wasp damage.
Why not grow some spring bulbs under the tree as you will still be keeping the ground clear for most of the year, and the bulbs will add interest and flowers before the apple leaves come out. They wouldn't compete with the tree in the same way as ground cover plants or other plants and shrubs.

Did the plastic eagles work, Maguerite? It's just that here one of our (French) neighbours hangs a plastic hawk in his cherry tree, and claims it keeps the birds away. I've often thought of trying it when I find a suitable plastic bird somewhere as the birds always get our cherries before we do.

OK, thanks for the advice - the apples aren't that nice actually, but perhaps taking care of the tree for a season might change that . . . .will get mulching and weeding!

And I thought blackbirds were bad!

I think saying that you shouldn't plant anything under an apple tree and that they NEED to be sprayed and fertilised is a bit extreme. I've worked at an 15 acre orchard for six years and it is clear that trees with nothing growing is not a great option. If you don't underplant the you either have to spray herbicide under them this creates bare soil which is unprotected from erosion, evapouration etc and. If the soil is not at it's best then the tree will be stressed leading to poor fruiting and susceptability to diseases and pest. If you don't spray herbicide then you will get grass growing underneath which isn't good for young trees as it interferes with the roots (old trees isn't so much of a problem as they have done most of their growing already). You could try planting beneficial plants around the tree, deep rooting plants and bulbs are best because they don't interfer with the trees root system. Therefore Comfrey and chives are two good options. Comfrey brings pottasium and other nutrients up from the subsoil and puts it into the top soil and also you can cut it four times a year. Chives are said to prevent apple scab after three years and also bring beneficial insects into the vicinity of the apple tree (in fact all alliums are good but chives are the best). Mint is another good one for letting grow under a tree (if its a small tree then maybe something like penny royal would be best) it's aroma is beneficial to the trees health, it covers the ground excellently and it also attracts beneficial insects.

If you have bad soil then comfrey is a must!