Thanks Astralite - I have unchecked S/PDIF check box which is on a popup from Control Panel/Sounds & Audio Devices/Volume tab/Device Volume - Advanced/Master Volume/Advanced. After rebooting, there was no change to the mono output.
What I find oddest is that it works in stereo using the internal speakers. I would have thought that, if it was the card, then everything would output in mono.
I had another moment of brilliance - get down the old PC speakers from the attic and try those. Yes, it worked but still it outputs in mono. That makes me fairly certain it isn't the stereo or the cable but something regarding the hardware or software setup in the notebook.
Anyway, thanks everyone for your help. Not sure what to do now - maybe get the USB soundcard? Seems an expensive workaround though when the solution could be, tantalisingly, within reach...somehow!