The construction near GZ was used as part of the Defence of Britain in WWII and a strategic situation for watching the coastline West from Budleigh Salterton for quite some way. The defences in question have now been put to most suitable peacetime use as a bat cave. Though no Batman & Robin, the Riddler has set a puzzle to solve which will lead you to the cache.
At the above co-ordinates you'll find the Grid Reference for South Farm, SY ABC DEF.
(Unfortunately, the construction for the original clue was demolished, and we've been told there is no intention to replace it!)
The final location is at: N50° (C/2) B . D E (C+E) W003° (D-F) (D+A) . (C/2) (F-E) (C-E)
You are looking for a decon container - you can ignore the hazard warning on the lid. Please ensure the cache is well camouflaged on all sides and securely closed when replacing in its hidey hole.