Hmm, odd - the encoder fault was not there today, maybe a PC reboot or complete shutdown fixed it? Been knocking seven bells out of it all morning and it's not returned so I'm happy with that.
Next I fitted up the Spindle VFD, expecting serious issues with my complex manual/auto speed control idea. I was 100% amazed that I have perfect control of speed from 40rpm to 3000rpm within about 25rpm tolerance! It tells me to choose back-gear or high-gear and high or low speed pulley setting depending on speed chosen.
The only issue it does have is that if it is set for say 100rpm in back-gear low range, and the code calls for 2000rpm in high-gear high range, it will spin up the spindle and then tell me to change gear - this cannot be done without smashing the drive.
I have all the code in SpindleSpeed.m1s - can it also stop the spindle and restart it after pressing enter without wrecking the code flow etc ??
Picture of my bench-mess below

Useful to gain an idea of cabinet size too.
A short video here...
https://youtu.be/RiXxmCpC-R0