Latest report:

I couldn't find the setting in MACH4 for the Dir Active Low (not in Output Signals, nor Axis Mapping, Motor Tuning tabs in Config, nor in the CS-LABS plug-in which is where I kind of expected to be hiding if its in there). I wonder if the CS-LABs guys need to write it into their plug-in for MACH4? Also CS-LABS have released a new version, but I figured I had enough variables to contend with in one night :)

Anyway, having lessened the ghetto factor recently on my control cabinet, whipping the drives out on their DIN mounts and putting a jumper on J3, seems to have worked a treat and the same toolpath that was resulting in a 5mm unwanted offset is now spot on, so thanks again chaps for the advice.

One thing that is still very ghetto on my machine is the belt tensioning "axle" and sprocket arrangement, which was hastily put on the machine 7 years ago when I put the 2nd ballscrew on the X axis. That's probably the source of the slack alice coupler/pulley sound - something I should have done something about really by now The Z Axis I'll investigate...