Thanks for the reply.
I tried this and still no fix. However, I've got got some more info which might help others.
I found it was insert effects on the vocals that caused this issue. Disabling all effects on the vocal track, fixed not only the vocal LED but the Master bus LED too.
So then I went, one step further; I DELETED all insert effects on the vocal track and re-applied the effects, saved the project and re-opened the project and this seems to have fixed my problem. Maybe there was a glitch in the file at some point when I originally added the effects.
If anything changes I will update this topic.
Many thanks