This is not legal advice, rather nature advice
In our old house, the roots from a weeping willow tree at one end of our property were growing underneath our brick patio and causing the bricks to buckle. Much like this sycamore is doing to your wall. We hired a tree guy (my mind's not quite awake yet. Landscaper, I guess he was?) who came in and severed the root from the tree and then pulled it out. We were then able to pick out the dozen or so bricks that had buckled and then lay them in the pation again. Everything was fine then.
Maybe you could try that? It cost us about $120.00 and it's not a permanent fix, but since those type of trees grow slowly it'll solve your problem for 10 years or so.
Good luck
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