Cheers jazz, I can move the axis by hand so don't think it is seized. Is it worth taking the belt off to rule out a mechanical bind? And is swapping the servo drives the best way to check if the drive is playing up?
I guess that the drive could have lost its current settings when the battery died in the servo drive, if that is where they are saved. But the z axis battery died too and I'm not getting an error on that axis.