I'm afraid what you found wasn't the gunpowder factory, that is further north as Swamp_donkey said.
You were here
http://www.flashearth.com/?lat=51.46518 ... =0&src=msa
The gunpowder factory is the link that SD has posted above. It's about 2 miles further north.
I've no idea what you found there, but it did prompt me to research the gunpowder factory a little, and I came up with the following.
There's not a lot of information about it because it closed in 1921.
I've dug up this mention on another forum
There was a pre-war gunpowder factory at Cliffe - the Curtis & Harvey works - it was actually used for storage rather than production from what I can make out. The book continues "To reduce possible danger from accidental explosions the storage areas were surrounded by earthen walls and the whole site by numerous plantd trees." The works closed in 1921.
There's also this photograph and information on the EH website.
http://viewfinder.english-heritage.org. ... &form=home
Maniac.