Just to close the loop on this - I've just hooked up a RS485 USB adapter to the controlling PC, linked it to the data lines on the VFD... then, like a lot of people, reversed the connections to the data lines, and with the help of the M3 plugin, have digital control of the spindle with NO wow/flutter. Clearly an analogue issue, somewhere between a cheap PSU and cheap BOB, but for now, digital control is the way forwards for me.