I would have thought that anything that is non military has been passed to Woodhorn, including tithe maps, so probably World War 1 stuff is the thing to go for at Kew. Of course what isn't there is probably held by the Northumberland Estates archive.
I'm off to Woodhorn tomorrow to have a spoach in the Charles Percy, solicitors records....seems there is something about Amble Gas Works and Kiln Bank House. Most likely the original documents for what is held at the Gas Archive.