Can anyone advise me how 'open' a throttle body butterfly should be at idle for a cable operated throttle body? Pictures speak a thousand words if you have any for reference.

Ive asked 2 mates and 1 said it should be closed fully and the other said it needs to be open a bit

Not sure what you mean? On my throttle body, I adjust the stop screw to set the idle rpm I want?

It may depend on what type of idling control you have ? If you have an idle speed control valve the butterfly may well be totally closed, if there is no idle control then a small adjustable gap is likely.

As above. My BMW engined car has an idle control valve under the intake manifold which regulates air into the manifold at Idle and this is ECU controlled. At idle the throttle body is completely closed. I suspect any non ECU car would have a gap at idle.

Chris