Well, thanks to you both for your input. The machine seems to be solid enough, as it's so tiny it doesn't take so much to keep it fairly solid. It's using M8 threaded bar as leadscrews, but the anti-backlash device Zen had added seems to be working well as far as I can tell. I was just making sure that it wasn't my noobie knowledge of the software, primarily. I'll keep battering away, and keep an eye on any mechanical problems. I've decided that using Neale's workaround of Vectric's allowance box (0.2 for outside & -0.1 for inside seems to do the trick).
Thanks to all, I'll be posting if I get any new info.
Of course, all my mates now have personalised 3mm ply keyfobs...
Cheers, all.