So I found Mach Loader (had apparently been ignoring it on the desktop for years), and there were a few mill profiles listed, but none of them worked. I did find in that root folder a file called "Mach 3 Mill.xml-GOOD" from 2010, which could only have been put there by me, so I copied and used that and it works again. Only thing I changed recently was to reduce the max speed of the X-axis, so that's already re-corrected. Much thanks!
Neil.