Thanks for all the replies. The inverter doesn't have speed control from the front panel. According to the manual AIC and AI2 are already configured and certainly if I connect a 1.5v battery between com and AI2 the inverter runs at about 15% as one might expect. If connect directly between +10 and AI2 the inverter runs at full speed, again as expected. So no problem with the inverter configuration.

Having played around with Mach3 and thought more about this two things occur to me:

1. I can get Mach3 to switch the spindle relay on, but the spindle DRO does not display an RPM and altering the speed doesn't alter any of the numbers from 0. Surely, even with nothing connected it should show values if it is generating pulses?
2. The inverter is looking for 0-10v between com and AI2, but where does the BOB get 10v from if it only has a 5v power supply? Do I need the 12-24v supply connected? I can take 24v from the inverter.