In the last months of 1110 a strange army appeared in Northern England. It was first heard of near a place called Penlaw some twenty or thirty miles north-west of Newcastle. No one could say where it had come from –it was generally supposed to be an invasion of Scots or Danes or perhaps even of French. By early December the army had taken Newcastle and Durham and was riding west. It came to Allendale, a small stone settlement that stands high among the hills of Northumbria, and camped one night on the edge of a moor outside the town.
From Jonathan Strange and Mr Norrell by Susanna Clarke.
The intriguing thing about the name Penlaw is that is apparently just about the only invented place name in the 1000 odd pages of the novel:
As a resident of Newcastle and descendent of Northumbrians this piqued my curiosity. Since there is no such place as Penlaw 20-30 miles north-west of Newcastle I wondered whether it might be some kind of philological code.
Pen = the name of a female swan; Law is a dialect word for hill
Which gives us Swan Hill.
Swan Hill turns out to be the name of a farm cottage in Longhorsley, which is a village... 20-30 miles north-west of Newcastle.
So my guess is that Susanna Clarke spent some time at Swan Hill farm or cottage at some point, and maybe that was where she had the idea of the Raven King's army, and decided to put it into the novel but in a 'pied' form (as a code or puzzle understandable by initiates only), or something...