Apparently there's a book written years ago for American's wanting to act in other accents including cockney. Suffice to say it's utter tripe and no doubt Dick used that very book.
The thing I hate about DM is that Dick basically uses it as a platform to further his family's career. He's an executive producer on the show and his son also plays his son on the show. Then in more recent series there have been another 2-3 Van Dykes also starring in the show. None of them can act. Not without being cheesy anyway. Quincy is much better. The cheekiest coroner around and a role model for those nearing their pension years. You seen the number of birds under 40 he pulls on that show? Respect is due to the old guy.