I've given up for now. it's too damn hot.
My tailstock was about 0.004"high and I've ignored my own advice and skimmed the tailstock adjustment joining faces on the mill and got it to about a thou which I will now ignore. The PITA is that my home made lever lock has got out of adjustment and I can't find the sweet spot 
What else have I done:
test – target – actual (all converted to thou)
Work spindle centre point for true running 0.0004 0.00025
Centring sleeve (chuck register?) for true running 0.0004 <0.0001
Work spindle axial slip measured at 2 points displaced by 180 degrees 0.0004 0.0002
Lead screw axial slip 0.004 none detectable
Lathe turns round within 0.0004 no out of round detectable
I know it failed the faces concave by no more than 0.008 in 12" test as supplied (there was a batch of early machines that suffered from this) but I cured this some time ago by skimming the cross slide dovetails.
The rest can wait…
Neil