I have now measured the 0-10v signal but only by disconnecting the VFD?

Odd.

Is the a way of editing linearity.dat manually?
Does anyone know whats in it?

I have listed volts to speed requested now and also speed requested to speed obtained.

Obviously i can use a manual lookup table but thats  not the point

SUCCESS!

I changed the VFD from the Huanyang to a UK Schneider Altivar12 unit.

It measures correctly now, the motor sounds better, smoother, draws less current. Sensorless-Vector seems to be unstoppable even at 100rpm!

According to the tech guy at the suppliers, with sensorless-vector drives it is 100% vital to run an auto-tune sequence once all the parameters are set.

I also managed to calibrate the speed to Mach3 successfully

In the end, all it took was taking the minimum spindle pulley to -6500 and max to +24000 - I now have usable calibration from 5,000 to 24,000 rpm to within 200 rpm.