Thanks for your thoughts, much appreciated. All input helps as we are pretty stumped.
At this stage I can't put up a pic until my post count gets up to 8.
I agree that posting in the tech forum may reach a more specific audience, the rules around non-pros posting in the professional forums looked pretty clear though. Certainly happy for the mods to move it over though if they consider it appropriate.
I agree that there may be a pin/screw some place but we have been all over the thing. As the nut incorporates what appears to be a spiral oil slinger we'll run a sharp probe around the grooves to see if there's something hidden in there.
Thanks again, cheers.