Thanks gang - much appreciated all the help and suggestions on here.

Yes, the platter spun fine with power off. Seemed level and no obvious problems.

Thanks for the tip, Robbie. Probably right as I can't think of anything else it can be.
I will try again to re-fit and fiddle with the MNB next time I have a free couple of hours to dedicate to it.

Will report progress when it happens!