Thank you for taking the time to look at this Myster. What I think brought this question to the surface was the mystery as to where the stairs to the 1st floor office originated? You know the ones which can just be seen in the cubby hole in the downstairs office.
Do you think theses stairs are accessed by an external door on the north side of the house?
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Short answer No..... you enter the dark-brown Back Door into a small hallway.., on the right a door to the kitchen with the Aga..., on the left a door to the Laundry although no door is drawn on Bamber's plan..., next to it on the left a door into Nevill's den/Back Office (this is the room where the photo looking back into the hallway was taken..., you can just make out a chair and coat rack on the back wall) and looking straight ahead in the hallway the bottom of the stairs which give access to the Upper Office. So access to this staircase is entirely internal.
The question I put in an earlier thread was whether there was open access to the bedroom passage from the Upper Office, or whether it was closed off to keep the two functions, residential and commercial separate.
Obviously if it was open then this staircase could have been used by the assailant to get to the first floor without disturbing the family or the dog, rather than having to use the main hall stair where risk of detection would be greater.