HI, all, sorry I didn't get back to you yesterday like I promised, but something always gets in the way. Particular thanks to Jason for giving up his income from MEW in order to help on this forum!
Firstly, not done it yet thought the vice was too big. It is a beast but …
You can mill across the whole span of the vice from full in

To fully out

So I'm happy that I've not gone mad from that perspective. True with the jaws in their alternative positions you can get to the rearmost, but are way short of the frontmost. Still it looks like I could hold an 8 inch wide piece and cover the entire surface.
On the accuracy: I started to clock various edges of the vice with the intent of a full report. But when I clocked the back face and found that it was absolutely in in line, I stopped and considered.
I took the fixed jaw face off and found grit behind it. I clocked the front of the block that supports the jaw (which was milled rather than ground so a bit on the rough side) and basically that was true as well. There might have been 0.01mm variation, but it was hard to tell.
I cleaned up the fixed jaw and measured it with a micrometer with the following readings
top left – 14.312
bottom left – 14.321
top right – 14.302
bottom right – 14.295
so 0.015mm variation at most (No I've not tried the a matrix of points to see if there are other variances, thought that might be interesting. It also seems (to a naive novice) more inaccuracy than I would have expected from a ground item.
The floating jaw was similar (and also grubby behind)
I put the fixed jaw back on and clocked again. This was better. A variance of 0.04 or so (a bit under 2 though for the metric deniers!). Having said that, I couldn't work out where some of that came from.
Sadly, the need to make dinner called and I left it there.
I've not yet checked the Z trueness, but I'm encouraged./I will play around with the fixed jaw when I have some time again and see if I can get it even better.
The fixed jaw, though does seem very close to vertical. within the range of my finger indicator anyway.
Iain