Hi Mike,
thank you for continuing to help. Your suggestions seem to be the answer.
With the Display and Color Manager preferences as you indicate, I made up a file of 6 colour swatches (R,G,B and C,M,Y) in QuarkXPress 7 and annotated them with their AppleColor Meter readings (the Red, Green and Blue being 100% pure).
I then opened the file in QuarkXPress 8 using the same preferences and lo and behold, the AppleColor Meter readings were identical except for the Red swatch which had a Blue component of 0.4%.
If, however, you set the Monitor Profile to "Apple RGB" instead of "Adobe RGB", the AppleColor Meter readings are absolutely identical and the CMY colours are possibly more accurate.
Job done, I would say and confidence restored.
Thanks once again.