Denford Triac VMC with ATC Conversion / Fix

[andy_con]

I haven't no, you could spend more time trying to fix these machines than machining

That does sound more like a tacho or drive fault. Have you tried monitoring the tacho voltage, to see what it does when the speed changes?
If the tacho voltage is increasing with the motor speed, then it's most likely a drive fault, but if it drops first, then there's a tacho problem.