Ooh, just found this thread 
CheerfulYank, I have the DVD of North & South. I agree about Richard Armitage ... oooooh yeessssss. Must watch it later.
The author was born in Manchester, a city in the North of England, and the story centred on the industrial revolution there that she grew up in, if you didn't know already.
The industrial revolution wasn't just confined to Manchester, it covered a very large area. Some of the cotton factories like the one in the film were based in Lancashire and Yorkshire, 2 connecting counties to Manchester, but which are a good distance away.
The accent used in N&S is a broad Northern accent that covers the Nothernmost parts of the very large area as above, so anybody from these parts can relate to the accent. So yes, the accent exists, yet all of the above places have their own accents too.
Hope that makes sense 