Thanks, I'll give that a try. I'd read the same procedure elsewhere, but wanted to make sure it wasn't going to radically move one end of the gantry out of alignment.
Yes, I did manage to fix the other problem. The Y-axis rod had slipped out of the bottom of both the front and back rod retainers which resulted in the rear retainer canting slightly forward (less than 1mm). Unfortunately this was just enough to make it miss the limit switch, which led to a few painful moments of my clamouring for the off switch.