I [may] have come up with an answer to this:
I have on my iMac a copy of VirtualBox which allows me to run Windows (tm) in a virtual environment on the iMac. I use this as a final resort for those things that I can't do, and can't replicate in the Mac OS, such as the speed camera locator update software which is Windows only.
I decided to try the PC version of HTC Sync and see if it worked. I copied the software from the internal drive of the HTC Sensation into the 'PC' and installed it. After the numerous required reboots I tried to sync the HTC with the PC version of Sync. This failed, but I was notified that an update was available for the PC Sync. I downloaded and installed this. After another reboot, I tried again. This time the HTC Sensation was recognised and tried to sync. As I don't have iTunes, Outlook or any of the required programs on the 'PC' I couldn't sync anything, but it proved that the 'phone side of things was working.
I then went back to the iMac and reinstalled the HTC Sync (Mac) software. This then located the 'phone immediately and once I set it up as before, I was able to sync almost everything that I had done prior to the ICS upgrade. For some reason, it is refusing to work with the Calendar and stops if this is selected, but everything else seems to be working as before.
Here's hoping it continues to do so...
It may be that the PC version re-wrote some corrupt settings that the Mac version couldn't, or perhaps the PC version replaced a driver App on the 'phone, either way, it seems to have fixed it. If you have a friend that has a PC, it may be worth a try?