Thanks for the quick answers.
I am thinking along the lines of aircraft performance, because at FL115 I can get over the top, where as staying at FL95 would mean a possible quick climb needed, or a very quick self-taught lesson on Chandelles in tight spaces. I prefer over the top
Yep, just realised myself that Article 54 applies to public transport, not private ops. So thanks for the advice on the pulse oximeter, I will definitely get one of those. I have also seen the Oxygen Pulse machines, which do not need O2 bottles to be refilled - any experience with those? I might pick one up next time I am in the states (assuming I am correct that the EASA certification for O2 equipment has actually been dropped in line with what I have read today)
To answer the point about which regs apply, this is the part where I am slightly nervous and edging on the safe position. I have had a fair few beers at the flying club discussing whether to swap my UK-Issued for a DE-Issued... (Many reaons not to, 1 is that work is a temporary contract here and so I will return to Blighty at some point and 2 is the beloved IMCr). The main topic of the debate has been how rules apply. Yes, German rules for sure because it is a D-Reg, but do the UK ones apply as well because I am a UK-PPL licence holder. A way to try to illusrate what I am saying is that I am told (still need to find the reference) that German PPLs are limited to below FL125 without Oxygen, but if UK had set the limit to FL100, which would apply... just as much as if I am G-Reg in UK I can fly IFR in both VMC/IMC, but if I flew a D-Reg into UK airspace could I do the same probably not; even though we now have an "EASA issued IR(A-Restricted UK)" or whatever it is going to be called this week.
I take your point about semi-circular vs quadrantal rule, but these are not exceeding any human factor limit placed by the licence. In the debates and conversations at the club we are sort of applying the "on an aircraft the rules of the state of registration apply unless the local regulations are more strict" principle... and wondering if this is relevant to licence issue as well.