Thank you for your reply, found the problem.............Noise
Supprised a bit as I thought mach might have seen a poor signal on what is basically just an on-off signal.
I know the source of the noise is the inverter but all the cables are shielded and grounded so don't know why I have problems with the original harness.
I ran a new cable to cure the problem, I had to do that before for the spindle index and encoder. I hope I don't have to do it for the 4 limit switches, they are just micro switches, not sensors so might be OK.
The lathe is a nice Denford Easyturn3 with the old combivert inverter. Waiting for a PWM to 0-10v add on to get full control of the spindle.
I have used the "Leafboy77" control which has so far been a great and easy board. Seems better (easier) than the smooth stepper I did my router table with.
It has 16 inputs + spindle RPM, all with indicator led's, which gives plenty of scope right off the board. I would give it a big + to anyone thinking of using one.
John