Okay - problem solved.
Turns out it's to do with µm/pulse as set up in the manufacturers-settings under options.
There's a little calculator which asks "what you wanted" and "what you got" (in my case 50m vs 62m)... it calculates what the µm/pulse should be. (in my case about '8')
Which is not as good as knowing the exact number - so I dug up the syscfg.ini from the old Leetro board... found everything to do with "pulse" then chose one which was closest to ,008. Leetro does mm, Lasercad does µm)
And lo : 8.11547
is the new µm/pulse value. Perfect.
There's a thread here
Rotary Attachment -need Guidance [Archive] - Sawmill Creek Woodworking Community on how to calculate this number... if you know the stepper/driver specs. I didn't, so I cheated.