Thanks for all the answers so far. It may be that UK airlines will need to be more stringent in their examination of the risks involved in operating an empty aircraft with a minimum crew of 2 pilots? A very interesting suggestion about a single pilot (pee/tea break for the other pilot for example, or cabin patrol etc) donning an EROS mask. Another suggested elsewhere that pilots entering an underfloor area should always carry/wear a portable O2 set. I don't remember that being pushed out as an SOP at anytime. Very good idea though.