Bagpuss, I can tell you that accents change over a very short distance, would you believe around two miles.
I went to a country school about two miles from the edge of the my home Town in Ireland, and I was regarded as a "townie" and my rural school friends were "country-hacks".
The same applies the world over, I once asked a German about this and he gave a similar answer. Ard Mhacha.