Boki_Srb, Thankyou so much for your help.

You are right, this is something strange with the head unit itself.

I added the lines Midnight Blue suggested to the system txt and it has made a difference in the way that NG is louder, but still out of one speaker.

However, I found that by adjusting the mixer I have got the radio fading and NG quite clear out of the front right speaker, which for my side of the road is correct.

So, its sort of a result which I can live quite happily with. Oddly though, if I have NG running with no radio or music it comes out of all 4 speakers. As you said, anybodys guess!.