Incomprehensible - or even just strong - accents tend to be associated with major industrial connurbations. Northumbrians aren't particularly difficult to follow, but Jordies... Or, to give a Stateside example, think of the Bronx or Queens accent. Birmingham, which is not northern, whatever Londoners might think, has Brum, which can be difficult to the newcomer. From the Scottish end of the country the Highlands accent is easy whereas Glasgae is next to inpenetrable to the outsider.
The reason southern British accents tend to be easier is that, London apart, there are very few major industrial towns like Liverpool, Manchester or Newcastle. Indeed, when thinking about southern accents my mind automatically thinks fo Darset which is distinctly rural, and therefore easy.
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Columb Healy