App Store hanging on upgrade to Mountain Lion from 10.6.8

I've got a Mac Pro (shown as version 3,1 in System Profiler) which is running 10.6.8 Server and I'm trying to get it to upgrade to Mountain Lion.

Unfortunately when I click on the "Buy App" button - or even if I click on the "Updates" button - the App Store application spins the progress icon for a little bit, but then hangs with the basketball of death. In the end I have to force-quit it - even leaving it for several hours doesn't help.

I've tried booting the Mac in to safe mode, resetting the NVRAM and similar - all to no avail. The host can reach the internet with no issues - I can see the initial app page, find the Mac OSX Mountain Lion page, etc. Its only after I've clicked the button to purchase the software that it hangs. I've tried booting it into the Apple Hardware Test, via holding down D at bootup, but unfortunately that doesn't seem to work.

Anyone got any ideas of how to proceed from here ? I've got a sinking feeling that the host will end up just staying at 10.6.8!