Brendan,
That is really helpful, and gives me hope!
I already screw the camera directly onto the diagonal, but like you the 1.25″ nose piece on the diagonal needs to be pulled out of the focuser to achieve focus, and I wonder if that is where some tilt is being introduced. I will explore, and I have a much longer 1.25″ nose which may help.
All good advice, I am very grateful.
James