Thanks you very much for the replies!
The part I was struggling with is why we use Mach number at all. I understand the relationship between IAS, CAS, TAS, air density, altitude etc but did not understand why the transition from IAS to Mach number depending on altitude - I presume there is a straight relationship between altitude and IAS / TAS would hold so why bother with Mach number?
I'm not sure that particualr penny has dropped yet even after reading the excellent reply from Keith Williams several times (sorry Keith its more me than you).