It's always cheers me up to see people posting about Northumberland sites. Your pictures of Little Church rock are intriguing.
Thanks for the info about possibly etymology of 'Duergar' too. I don't really know any welsh, but 'dwr' as water is interesting. I think you're right about the Brythonic angle, the immediate post roman peoples of what's now central Northumberland are referred to as Brynaich, which seems to hint strongly in the same direction.
I'm off to ponder the water connection. I can't immediately connect it to dour Hill or Dues Hill, both Northumbrian place with ancient sites, but it does connect with Dewley hill, where there is an (early bronze age?) barrow and a small lake at the ehad of a burn.
Welcome to TMA.
Hob