NW1...thanks for the Concorde insight - most interesting...it serves to highlight the irrelevance of the Mach Crit value for simple drivers such as us.
BOAC...thanks for the esoterica, this is, after all the reason why we visit this forum.
My observation of most ATPL level aerodynamic models is that they are, in the main, 2 dimensional - and thank god for that!. Great for teaching but risky business to extrapolate to real world cases.
Personally, I prefer the definition of Mach Crit to be when the pilot first becomes aware of its effects thus placing its value above Mmo in most (1G) cases - which may be why poor old flamgat will probably never learn the answer to his question.