Okay, I think that I've got it. It is, I think, a Sector Operating Centre for the ROTOR radar system but was made redundant in 1960. Between 1966 and 1968 it was taken over by the Home Office for incorporation into the Regional Government organisation as a Sub-Regional Control (think Kelvedon lite) and was refurbished between 1976 and 1981. It was sold in 1996.The owner of the cottage didn't know much about it. She thought it was a regional civilian command centre, like Kelvedon in its later role.
(Page 109 of "Cold War Secret Nuclear Bunkers" by Nick McCamley)