my co-pilot fumbled the ( new to him ) INS leaving Singapore for Australia, and insisted that I flew a heading of 310 (M) That's what's wrong. Too much reliance on new tools without understanding the old, basic principles first.
Without looking on a map, I reckon he got the reciprocal. If so, he needs to look at a map
I don't think this is related to navigation, as such. But it's easily done - I could not have told you where many of the various European countries were relative to each other, until I started flying there.