Thank you, that's cleared it up perfectly.

I was going to run my VFD on a separate socket outside of my electronics enclosure - so I can access it and because I assume it will generate heat and suck up a lot of current. The only part of the spindle system I was going to run through my control box is the connection between the VFD and the PC (Using a RS232 to RS485 Converter and grounding the cable shield inside the box - to the star ground). Are you suggesting that I should still bring over the earth wire from the VFD into my control box even though it's on a different plug? Should I do that instead of or as well as grounding it at its own plug? I thought I had to use a 15amp RCD breaker between the RFD and the power?

Also, since you've answered so clearly and generously, one more thing - I am guessing the spindle height touch plate I intend to use needs to be connected to one of the BOB inputs and the spindle earthed to the GND on the BOB? - even though it is also going to be Earthed via the star ground to the control box through the earth to the machine? Does that mean I should only connect the BOB ground temporarily while I do the setup? Otherwise the touchplate would be grounded (through my machine) to the star ground AND to the ground on my BOB. Hope that makes sense.