Latches are designed to stop unauthorised entry, not exit. How hard would it be to put a wire on the handle and pull it down from outside with the wire detaching and being pulled out through the gap between the window and the frame. And for all anyone knows he got out from an upstairs window using a ladder.
Bamber is so deluded he thinks everyone will fall for his rantings.
Piano, guitar or any thin wire, even strong linen thread used to secure the side fastener. Bottom stays are easily pre-adjusted to drop into position when a window is shut from outside, especially if well-used and worn. Having a solid-fuel AGA on constantly could also mean that the window was always being opened for ventilation, wearing down both stay and side fastener.
I don't know if E.P. pursued a ladder theory. Susan Burgess, one of Sheila's friends, recalled (in a w/s) that in her teens she "escaped" from her bedroom (twins' room) above the kitchen using a ladder, to visit Maldon and Colchester after dark. So it might have given JB the idea. A lot more work involved than a downstairs window; placing it and then hiding it when the job was done; sliding the lower sash back down while balancing on the rungs, so he'd need a head for heights. The top sash in the main bedroom was left partly open, so no need to fasten it either. If asked, Bamber could say his parents left it open for ventilation on hot summer nights.
