I've seen some other threads about optical audio issues but none seem to match my unusual one.
TV is a Samsung tu7100 43".
I think that when I first connected up my speakers things seemed to work as expected. The next time I used the TV it didn't seem to be working. I switched the settings back to "tv speakers" and realised sound was playing out of both TV and optical out. This theme seems to persist - when set to "tv speakers" it plays out of both, and when set to "optical" I get no sound at all.
I'm hesitant to reset the TV due to accounts and what not but I will if it'd be useful.