Given the scenario of direct head on confirmed by range closing,bearing not changing, why should anyone contemplate a left turn,as bookworm has suggested ? No, seriously, tell me, there is probably a good reason which my superficial approach has missed.!
Notwithstanding Rules of the Air rights of way - you are entitled to do anything to avoid immediate danger - just because you have seen him doesn't mean he has seen you!
Also emergency (ie avoidance) turns are not in the PPL syllabus whereas the military do teach them to students. I think ETs should be in the PPL syllabus - there was a thread on Pprune a while ago on this topic.