I visited this post on Sunday evening. Just outside Ashover (Derbyshire) on a hill called The Fabrick. A very beautiful location.
The hatch was locked internally so no access was possible. I spoke to a local walking his dog who told me that " The Doomsday bunker was pretty trashed down there". The shaft and ventilation shaft were once clad in derbyshire stone But somebody had nicked the cladding.
Worth going up there just for the view. a small square drive key is required to unlock the post. I'll be back.