God Feral , did you see that actual Australian guy in Prime Suspect 73 trying to do London? It was the most awful thing ever...
If I remember rightly, Arnott's backplot makes it essential that he is form somewhere very specific in the south of England. That's in series one but, what with everything being so complex , I have completely forgotten what, where, why. Odd, with all the other geographic vagueness, but that's why. I threaten vaguely dark actions against anyone who criticises my Martin, having followed him since about 2005.... he is the reason I watch (or was at first)
Adrian Dunbar's diction makes up for all of it! 
I love Line of Duty (and Broadchurch) because NOONE MUMBLES! It's lovely to hear David Tennant actually being Scottish, too : it annoyed me all the way through Doctor Who that he did English, when it matters not one bit to the character!
think everyone thinks anyone can do West Country : don't think Julie Hesmonhalgh's is very good