Complex sentence: It was originally written, as certain peculiarities of construction and vocabulary clearly show, somewhere in northern England, but of the author nothing can be learnt except the fact, which he himself tells us, that he was a cleric.
Split into simpler sentences: It was originally written, as certain peculiarities of construction and vocabulary clearly show, somewhere in northern England. However nothing can be learnt about the author except the fact, which he tells the reader himself, that he was a cleric.