Fixed! I'm unstuffed.
I managed to get hold of someone helpful from the programming team for tv pack at Microsoft on another place. It turns out that in the development process, Ms altered the audio codec to sync it with h264 (hi def) video codec. In testing, they found that the new audio codec struggled with higher bit rate audio processing.
So where I had my 24 bit soundcard configured to 24bit output in Windows, I've had to throttle it back to 16 bit to restore the 5.1 audio. In practice, the difference is not apparent to the mk1 ear.
Who knows, someone may come along after me with the same problem & read this & go away happy. It could happen...