Thanks again for both your replies.
Completed the loopback test and it is not communicating ( I checked the Uno R3 on the Windows XP machine and it successfully communicated using Serial Monitor ).
I doubled checked both USB ports on the Mac using the System Information Utility to make sure that it was "seeing" the Uno R3 and it is. Although on one of the ports ( Bus Number 0xfd ) it doesn't retrieve all of the device information compared to the other port ( Bus Number 0xfa ) - is that important ?
Are there any other tests that you can recommend to help pin point the problem ?