Pruning apple tree advice please

[Pineberry]

I have an apple tree that I inherited when got an allotment. It is kind of lopsided. The weight of the branches are pulling the tree down to that side ( and that's without of any fruits). It is not even sitting firm in the ground. I had use a pot full of soil to support its trunk.
Can someone give me an advice on how best to prune it so sort try to balance it as a result.

It is not a tall tree just over 4' I think. The apples are big so probably a cooker.

[FB.]

Maybe have a look at its underlying structure during winter when it's dormant to see what could be done.
I think the best you'll be likely to be able to do is prop up the leaning trunk by wedging a large post underneath it (with a piece of rubber to reduce chafing and reduce bark damage) and let the tree carry on. It might also be possible to graft another tree, trunk or piece of rootstock to it on the other side, to make an archway.

I'm in your area often enough, so in the autumn I don't mind meeting you at your allotment to have a closer look at the tree and a chat about what might be achievable.

[Pineberry]

Thank you, that would be great. The grafting part I would need help with, I presume that would be a major thing, or I might make it worse instead.