Many thanks for all replies, really interesting subject.
Just one purely hypothetical question on the subject of cruising altitudes if I may...
Assuming just for a moment that neither engines nor wings are a limiting factor in determining max cruising altitude, does there come a point where the limiting factor is the increasing differential air pressure with increasing altitude between the interior and exterior of the fuselage pressure vessel?
To put it crudely, "don't taker her any higher or she'll burst"!