Grassthorpe Hideout.
Currently the only info we have is about a secret hideout at Grassthorpe. This was documented by the Defence Of Britain Project, undertaken by the Council for British Archaeology between April 1995 to March 2002, when nearly 20,000 twentieth century military sites in the United Kingdom were recorded by an army of some 600 volunteers.
See the record here which states:
Location: In the roof space of the old manor house at Grassthorpe.
OSGB Grid ref. - SK 795 678
Lat/Lon (OSGB36) - 000 48 34 W 53 12 03 N
We would love to update this with any more information that can be shared with us.