As always, a very detailed diagnosis.
the focuser does have tilt correction grub screws, but they are so difficult to manipulate, it makes them very impractical.
The image train is ZWO 1600mm Pro, zwo 5 position fw, zwo OAG and a 16.5mm spacer which attaches to the scope.
I’ve just been out and done a full inspection of the scope & image train. Made sure there is no slop and everything is tight. I’ll give it a go later on and see what happens.
As said, I can nearly always crop out the bottom of the frame and still end up with a good image composition. I guess reading how it should be vs how it actually is is not always true!