My CNC Plasma Cutter has developed a peculiarity

It is driven by a customised Mach3 screen set with not much of the original Mach3 screen being retained.
A/ If I move the cutting head to what I want to be X0,Y0 and press a button 'Zero X & Y' the displayed co-ordinates indeed set to zero.
B/ If then I load a cnc file generated by Sheetcam, it displays an image of the part, but shifted in X by 100 mm
C/ If I cut he part it is cut offset by 100 mm in X
It's as though an offset is being added to just X.
Now I've rolled the setup back by 22 jobs which is all that Mach3 saves, and it's still the same - though it IS possible I've had this problem for more than the last 22 cuts. Now I'm not that conversant with Mach3 and anyway much of the support screens normally in Mach3 are stripped from this build so I'm a bit stumped how to proceed.
. . . I can still work with it WHEN I remember to set zero X back 100 mm but I'd dearly like to solve the issue!
. . . any suggestions?