So, I take it this is the 'back kitchen' according to the plan?

No, that's the kitchen where Nevill was found. The police came through the back door to the house, and turned right through another door into the kitchen. You can see the door through which they entered the kitchen in the top right of the photo.
Hi kaldin, the door EP came through into the kitchen is the door I would describe as top centre of the photo (just to the left of the old fashioned tea dispenser on the left hand side of the red aga surround).
It is in this corner of the room that Mike T suggests NB was sat (dead) in the chair prior to EP pushing the door open and NB then somehow being left in that "precarious position" propped on the overturned chair with his head positioned in the coal scuttle.
It may also be of interest that you can see the window edge (top left of photo) and if NB was sat in the chair in the corner of that room it may, as Mike T has suggested on many occasions, be difficult to see anyone in that position from the window!
back on topic: this is always going to be difficult to establish (as with most things here) as it simply depends on whether one believes in what JB says or not!
If we start from a misinformed position we will nearly always end up in wrong position
An intriguing question (amongst loads) I would like answered is how did NB end up in that awful final position?