I reckon the Diary Room will be at the top of the stairs.
The button to access the Diary Room is downstairs shown in this picture:
The door in the background looks a little small/plain to be the entrance to the diary room, although it certainly could be as the DR is usually in that area I belive.
There appears to be a sort of landing upstairs and with rumours of the DR being upstairs, it could be to the left of the stairs, shown in the image.
(Of course, I could be totally wrong!)