The IGI entries are listed by county as specified by the LDS. This is not necessarily as one would expect from reading an atlas. There are many entries in Durham that should be listed under Northumberland. This is probably because the Bishopric of Durham covered most of Northumberland until 1884 (I think). So if you're looking for Northumberland entries you might want to skim through Durham as well.

Durham (299 entries)

Yorkshire (41 entries)

Northumberland (18 entries)

London (17 entries)

Rest of England (36 entries)