I did another test tonight with the table removed and checked with a square
how true the Y movement was. The results show a 0.4mm error over about 65mm.

I repeated yesterdays test and set the dial to +-0.03 over 140mm on the X axis. A 6inch square held in the vice as before and I get 0.62mm error over a Y movement of 120mm. From this I can work out the angle error between the saddle X and Y dovetails.

I now have two choices, either get someone to machine the X/Y channels true
or possibly get software to compensate. Can mach3 auto compensate for non true XY axis!!!