Following the second World War, the Royal Observer Corps which had been “the eyes and ears of the RAF” were briefly stood down, but were stood to again in 1947 to counter the threat of the Cold War. The role of the Corps changed from spotting aircraft to a new more technical one, that of Nuclear reporting, in preparedness for a possible nuclear attack. Between 1958 and 1968 a countrywide building programme resulted in a network of 1,563 underground monitoring posts, approximately eight miles apart, distributed throughout England, Scotland, Wales and Northern Ireland. Construction work on Threlkeld was started in 1958 and was operational until the Corps was once again stood down in 1991. The post was part of a “Cluster” of 3 posts, the other two being Penrith and Bassenthwaite, and would have reported nuclear explosions and the resulting fallout to Control at Carlisle. http://en.wikipedia.org/wiki/Royal_Observer_Corps_Monitoring_Post
The BFI have released a marvellous wartime film showing the aircraft tracking role of the ROC, our heritage. http://player.bfi.org.uk/film/watch-royal-observer-corps-1941/
As an aside, there seems to be a lot of confusion regarding the iconic piece of wartime ROC equipment.
This is a “Post Instrument”, yes that is the correct name, it is not a “Micklethwait”.
The “Micklethwait Height Corrector” (named after an Observer Micklethwait) is a later addition to the original Post instrument and comprises a sliding bar with a pointer and a height scale as highlighted in the picture below.
If the position of an aircraft over the ground was known (i.e. directly above an adjacent Post) then the Micklethwait pointer was moved to that position and the true height could be read from the Micklethwait scale and the Post Instrument correctly calibrated.