If you are correct it is no wonder I couldn't find the solution in Windows.
One thing though, I turned it on and it failed to boot into Windows. Not realising this I was just about to plug in the HDMI cable (thinking it had booted and was still trying to use the tv) when I turned it off. As I turned it back on I got the Repair screen, although I opted for booting normally and it has been alright since, apart from this problem that once connected to the tv it will not revert back.
I have really messed it up and am currently back on my main machine.
I inadvertently enabled both tv and monitor (seemingly as one screen rather than mirror) and was all over the place. I am running a System Restore to the one I created earlier, hoping that will at least get me back to where I was.
Once I achieve that I shall start looking for the key you mention.