Help me rejuvenate an old apple tree

So on top of a multitude of reasons to dislike our landlord, it appears her need to make everything look a certain way meant laying huge paving slabs and pebbles around a century-old apple tree.

The slabs are in a circle surrounding the tree, and they stop maybe a meter out from where the branches above them stop. After that, it's 5cm of pebbles to stop any benefit from rotting foliage.

It's got all kinds of signs of age- stripped bark here and there, knobbly bits and so forth. Now we may be leaving in six months, who knows, but I'd like to give it a feed.
I've been told the slabs have compacted the soil, preventing the tree from getting much out of the soil, so I thought I'd start there. I'll take off a few slabs, leave some gaps here and there for the tree roots.
I don't care for the fruits, I just like to give it the right ratio of nutrients for a fighting chance.

What do people recommend? Expensive, or cheap internet hacks welcome 
Is there anything I can make maybe from the mountain of lawn cuttings I've got?


Thanks