I can't see 20 sources Adam? the police said the windows were locked and secured and checked by several officers they said nothing about the secondary latch being unfastened on the kitchen window? which cannot be closed from outside.
Doesn't matter what the police said.
It just needs to be proved the kitchen window could be banged shut from outside.
Bamber was very helpful regarding the windows. Saying he knew how to get in & out of these.
Obviously he knew about the lockable from outside window as Julie knew.
David says the relatives told Julie but has never said why or how they alerted Bamber's girlfriend.