Yes, doesn't really help himself by announcing that incite, does he?!
Thank you both for the explanation of the window. I grew up with sash windows with rotating locks: it's interesting to see how easily they can be opened with a knife, but I would have though impossible to close again from the outside. I still wouldn't have thought it was easy to shut the casement window: although it might be theoretically possible on a new catch, the building looks like so many period buildings I've experienced where in reality it's had so many layers of paint it would be quite stiff, and it would also require a latch half way up the window to be pulled down: isn't this the window the police looked through, with the lights on whilst they were in darkness outside? If so, it would have been possible to see the second latch not in place even in outline from a slight distance.
As I posted elsewhere, is it possible SC was lying in wait for JC to arrive, knowing his preferred entrance through a downstairs window? The'd both been to boarding school - this kind of thing happens quite a lot, and I can't imagine JC was the only teenager from that house who maybe went off to the pub without his parents knowing. Even a murderer being arrogant would spot the stupidity of announcing he went in and out of the house through the windows if they've set the scene to be a murder-suicide by someone inside.
One new thing I've just learned from Lee's book (p.190) is that although all the downstair's windows were checked to be secure by DCI Taff Jones at around 9:15am, they had also been examined earlier by two police officers who found the shower room and kitchen fanlight to be unfastened. One of them, Sgt Stephen Golding stated in his w/s that he secured both so that they couldn't be moved but failed to inform his superiors.
So JB could have parked his bike when he arrived near the Shower room window on the opposite side of the house to where the labrador was kept in a barn, so as not to disturb it. Then exited by the same window a lot more easily than the one in the kitchen, again without causing the dog to bark, jumped straight onto the bike, headed back home, and took a chance that the police wouldn't discover that the window was left unfastened.
There was no need for Sheila to wait with a rifle in Nevill's back while he phoned his son, or near the Shower room window had she killed mum, dad and her sons in advance. It makes no sense for her to beckon her brother over from 3 miles away in the middle of the night simply to kill him, because if she was determined to commit suicide, inherited money would be no use to her.