OK, so a massive schoolboy error on my part - the optical out was working fine, but the cable had come loose at the back of the amp, which is inside a cabinet. Ugh. It's all working fine now - However, I did find something curious out. I was playing my Assault On Precinct 13 LD and it (according to the back cover) contains an isolated score and a commentary track on the analogue channels. Now, cycling through the audio channels when coming out of the optical out there is always sound on the channels, even when on analogue, but never the commentary or isolated score. This leads me to believe it does not actually output the analogue channels through the digital out - it just plays the digital stereo. Unfortunately I had to remove my RCA connection to my LD as I can out of inputs on my amp, but I will test this soon to verify this.

Thanks again for your help guys!