This is one of three underground divisional control rooms built
to regulate the flow of railway traffic by Southern Railway during
WW2.
As you approach the area, you'll see two blue doors, these are
the Northern entrance to the bunker. The cache is located by the
Southern entrance, next to the railway line and is only available
from the North.
Given recent postings, I've changed the location of the cache.
It's easier to spot, but harder to retrieve.