Okay. It seems to have been resolved. I manually removed all of the files from the PHATSYS directory and then reinstalled the firmware, VOIT and AAC plugins using DMS Tool 12.2. I choose to install unhacked firmware and choose not to flash the Keg. Plugged DMS into the Keg and worked perfectly!
I think the problem was me, as I'd tried to replicate the properties of the original 10GB DMS. When checking the properties of the original DMS using PMM, it shows that the AAC Plugin is installed. When the DMS Tool 12.2 installs the AAC Plugin, PMM doesn't indicate that the AAC Plugin is installed. During my first attempt at getting the new DMS to work, I followed advice found in another post on here, and I manually copied the AAC Plugin files from the 10GB DMS to the new 120GB DMS; PMM then showed that the plugin was installed but the DMS wouldn't work in the Keg.
Before manually removing the files in PHATSYS I did use the DMS Tool to try and repair the 120GB DMS, but each time it failed. I'm running Vista Business 32-Bit. Has anyone else experienced this and is there a solution?