Another strange point that emerges from Ann Eaton's long witness statement is the key to the back door which was kept in the coal shed. The housekeeper plonked it on the kitchen table on 11th Aug when she and Ann were at WHF, causing the ever-suspicious Ann to wonder why the police had had to break down a door to get in when Jeremy could simply have told them to use the key. She assumed Jeremy knew about it which is reasonable enough I guess. If he did then that would have been a better way in than via a window, all the business about hacksaw blades and sash windows etc would be irrelevant and his claim to have entered the house by window in the past rather odd.
Perhaps his parents never told him about the key. Pity Ann didn't go all Miss Marple on the housekeeper about it. Then we would know more.
There might have been more than just the two back door keys they knew of, but JB told police that his dad secured all the doors at night by key-locking, and bolting, and sure enough when Collins, the leader of the first TFU team peered through the back door keyhole, it was blocked. So they had no alternative but to batter down the door. On their first recce around WHF the police possibly tried all the doors and checked keyholes, including the front, dairy, and rear hall/scullery (5 on plan), and discovered that JB was, for once, telling the truth.
Bamber's kitchen window exit seems plausible. Although there are no internal pics of that large opening casement online that I know of, it was fastened by a side catch stiff enough to stay raised while a person climbed through the window, and which then dropped into a locked position (stuck at 8 o'clock according to JB's letter to one of the recent book authors) when the window was banged shut from outside. The noise might have caused Bruce the labrador in a barn to bark, but once JB had completed the crime there was nobody alive left to hear it, other than people such as the Foakes family living in the cottages on Pages Lane. But they were at some distance and might have slept through it anyway.
Another theory is that JB left the same way he entered, via the Washroom/Shower room window. There's some confusion as to whether it was secured or not. Taff Jones reportedly checked all downstairs windows and found them to be fastened, but a couple of officers who checked them earlier, found the shower room one to be unfastened and so refastened it. Then again, the same window might have been opened earlier still by the TFU as an escape route, or to let some wind blow through to disperse any sickening bloody smell coming from the kitchen.
Right... off out for the day. If there are more questions, anglolawyer, maybe others will contribute in the meantime.