Well, after posting this, having lunch, shoveling the driveway, replying to Phil's post and coming back to my shop, it of course is working now.
However, my PC had gone to sleep and my usual screensaver was not displayed - I just had a black screen. Upon reawakening I was going to switch the X and Y controllers to check for electrical problems but when I tested it, G1 Y1 was working. I've run through an entire Gcode test file with lots of movement with no problems. I'll just keep my eye on it to see if it happens again and report back one way or another.
cheers,
Michael