Update: I tried plugging the speakers into my phone and voila! the speakers worked fine. So it's not the speakers that are the issue, it's the line out socket on the back of the PC. I then plugged into a different socket and they work fine :)
My Logitech Z623 system plays sound fine (and I can change volume ok) through the LHS speaker and sub. But the RHS speaker only plays at a very faint volume and this doesn't change when I alter the volume.
I've pulled the RHS speaker casing off to have a look for anything obvious, but nothing...