Why didn't Jeremy cut the telephone wires with the hacksaw ? Silly boy,he didn't think when he set out to murder 5 people that all were capable of dialling 999. 
Silly boy , fancy showing the police after the murders how he got in and out of the house , and fancy leaving the hacksaw there as well . Very silly .
Oh but of course he did readily tell the police he and other members of the family did go in and out of the windows when they came in late - But the windows did not LOCK from the outside. Not as if he was hiding anything . Oh and he left a note for BW asking her to lock the windows as well. How kind.