Looking at that picture a large part of the trunk is missing. The piece lying on the ground looks a lot thinner than the stump. Might just be the angle but if that is the case someone has cut it up…
Its just the angle. The hinge is still visible on the felled part, and if they’d cut it up they’d have made a right mess over the wall/ground too.
I would start by looking at anyone who’s had a TPO slapped on them recently.
What you did there….. I see it!
I was being serious though, I would bet its the recent recipient of a tree preservation order from Northumberland council thats decided to cut down Northumberland’s most famous tree. It’s exactly the sort of thing someone from round these parts would do.