Jeez it's not like your sitting in some 60's turboprop, and the bloke is hand flying it. Your in an advanced jet airliner, with the autopilot engaged (the third man), a highly trained / vigilant pilot at the controls, ATC are watching you, TCAS is fitted, etc.
Also, w.r.t. 'after all is it not the case that heading and level instructions are confirmed with the other pilot before being input into the auto pilot'. Erhm, so just when are you suppposed to answer a call of nature ? e.g. I can see it now (somewhere over middle Italy, at FL330) "Roma please do not give us any ATC instructions for the next five minutes, whilst our Captain leaves the flightdeck to take a crap"...... get real !