Rules on boundaries and trees

[TropicalPotatoes]

I understand we can cut off anything that overhangs our land.

What if it's the main trunk of the tree?

You’re allowed to cut off overhanging branches providing you can do so without damaging the health of the tree. So, no, that wouldn’t extend to cutting the trunk.

Is it just big, or is it leaning right over to your side?

That could kill the tree or make it very unstable so no.

You can cut overhanging branches, but not to the extent that it actually harms the tree. Definitely not allowed to chop the trunk.
How much of it is actually over the boundary?

Was it already there before you moved in?