I had a really interesting situation a couple of decades ago when I was studying. I grew up in a rural part of the UK in the South West. The nearest train station was just over the county border, around 20 miles away.

One day I was waiting for the train, and there were two men talking: a vicar and his friend - both in their 50s. Clearly from that area. Even though I'd grown up in an area with a similar accent - less than 20 miles away - I could not understand a word they were saying.