the most impenetrable accent in England is broad Derbyshire - absolutely no contest.

I married a Derbyshire girl, whilst she was always understandable - her father's accent took me about two years to attune my lugholes to.

Fo the first six months I thought he had a friend called Wesley that he was always talking about.

In fact it was just the way they contract the first person pronoun,I or we, and run it into the next word. so you'd get something like:-

wes'ould gerrus skeats on.
(I had better hurry up)

Apparently HIS Dad talked real broad Derbyshire.