Guys, I've just picked up your posts (been out all day), will take a trip back into my workshop with a clear head (won't last long though!), I actually have high hopes that this will be sorted tonight, a couple of things I did late on last night was ....

1. Changed my Z resolution (from full step to 1/8 step...my Z 'pitch' is 1.5mm so steps per mm has gone from 133222.3 to 1066.6666)...at least this way i'm figuring, if I'm missing stepos then the error won't be so huge!

2. I calibrated my Z axis (not even sure whether this could affect the outcome, bt it was out.

Re the pcb kmaterial....clearly I've leveled the bed, but the copper board is double sided sticky taped down to some 5mm (flat) acrylic spoiler material...this in turn is clamped to the bed...so there is the possibility for an unplane copper work surface error...it shouldn't amount to what I was seeing which was 0.35mm over just 10mm (also, the fact that I took my V cutter down to 'just' scratch the copper & then tyraversed 10mm to Y10 ....the line disappeared half way along (since there's no ascent/descent going on here, then this must be board level error)

The CNC software itself can't be ruled out...this auto Z probing is a *very* recent feature in CNC-USB software.

Anyway, lets see what this evening brings!

Thanks for your support though....sometimes you just get a little close to the wood & can't see the trees blah blah.