Hmmm.... Whilst looking around I have just tried the EWS connectivity test again this morning (about 6 hours later) and it succeeded. It also showed successful using the downloaded Microsoft Connectivity Analyzer running on a local Windows 7 machine. I've not changed anything at my end.
However, the Mac is still unable to connect. A different Mac running Mountain Lion and Office 2011 without SP1 also is not successful in the exact same way