You were spot on Hood, I had a friend "helping" me a few days ago, and he fiddled with a lot of the settings trying to get the motors to work. He knows less than I do about MACH 3, and I know precious little. I told him I didn't think what he was doing would make any difference. I thought I had reversed all that he had done, but obviously missed his fiddling on the outputs page. The motors were still not working when I fixed the outputs page, so I tried checking the active lows in the DIR and STEP boxes, and when I set the STEP to low active they functioned correctly. It certainly is a steep learning curve when you start in CNC, but thanks to you I'm continuing my slow crawl up the slope.
Many, many thanks, Denis.