As a northerner living in exile (trying to convert the southerners of this land to the true path of tea, flat hats and whippets) I'm still trying to find the north/south line.
There seems to be two realistic opinions:
1. A line joining the Wash to the Avon
2. The North is regarded by some to be the counties within the C of E Province of York. (ie, Cheshire, Yorkshire and other counties further north.)
Personally, I prefer the latter. Somehow, I can't imagine counties like Herefordshire and Worcestershire as being in the North.