Thanks for your reply cbyard123. I tried various things from that thread but nothing worked. I did stumble across a thread which mentioned several people could only boot from Unibeast if they were installing to a SSD.
I'm using a OCZ Octane SSD. So I'm just going to try a standard HD to see if that works.
*Update*
Final sorted it out. OCZ Octane SSD firmware was not compatible with Mountain Lion. Apparently lots of Mac users had experienced the same problem. You must have OCZ firmware of 1.14.1 or higher.
With the new firmware installed, the system now boots and operates fine (including sound).
However, I did get occasionally/random kernel panics on startup. It appears to freeze at a slightly different place each time when I start up in verbose mode. This turned out to be a problem with VoodooHDA 0.2.7.3, so I took a gamble and used VoodooHDA 2.8.2 pkg installer. This appears to have cured the kernel panics.
So to recap. I started with standard bios, my Multibeast 5.2.1 options for this install were: The DSDT for the D975XBX2 mentioned earlier in this thread, VoodooHDA 0.2.7.3, IOAHCIBlockStorageInjector, Chimera, Default theme. Then I installed VoodooHDA 2.8.2 pkg installer.
Thanks for all your help, really appreciate it.