Leaning apple tree

Of course it's worth having a go - it's that or lose it, isn't it! Don't try to stake it by yourself, though - I can imagine your back going under the strain. Get someone to help you. Pick the rest of the apples. II would hammer a sturdy stake in - or even two if the tree is in a difficult position for one - and then rope the tree and heave it back up, then tie it firmly with those tree ties that you can get - the wide slightly flexible ones. (If the wall is in the way, you will have to try leaning on it and pushing rather than pulling). Lastly, firm the soil back down around the base. I do hope that it survives.

I've had exactly the same problem with a "patio" 2-in-1 apple tree I've planted out! Blasted rain soaking the ground and the winds we've had helping to push the d*** thing over!! Fortunately, mine is relatively close to a fence with *extremely* strong concrete posts so I've tied it to one of those (with a rubber strip between the tie and the trunk) and trampled the soil back down as best I can in the hope the roots will sort themselves out eventually. We'll hope and trust in nature together!!