During the war there were stories of planes suddenly dropping from these heights,with pilots falling unconscious, due to they supposed oxygen running out, and or freezing and fracturing of essential pipes. Sometimes pilots managed to wake up with just a few thousand feet spare, but sadly apparently many didn’t.

Any one know any more?

Thats it – in a nutshell, oxygen starvation is the biggest risk. Of course, so long as the oxygen system works properly inadverdant flight in to terrain (ie hitting the ground) is less of a risk due to the altitude. Checking reality, here in the UK, Permit catergory aircraft (ex military aircraft such as Spitfires and Mustangs etc) are only cleared for daylight VFR flight so gaining altitude without breaking the rules and getting down again may cause problems, oxygen or not!

It can be different in the USA of course, as most Mustangs are, for example, registered in the ‘Limited’ category which permits flight over 100% cloud cover (no visual contact with the ground). This can present its own risks – I recall three or four years ago two unfortunate people had to bale out of a P-51 named ‘Glamorous Glen’ after the engine stopped during a period when they were transiting over cloud which was over a forested area (no possibility of a forced landing).