. . . because of his description of how he could use a hacksaw blade to open the catch and enter and bang the window closed to make it look as though it was locked.
Where exactly can we read such a detailed description? I thought he had said he couldn't have fully secured a window shut from outside (or words to that effect).
This is why Jones thought the windows were all locked however, when Sergeant Stephen Golding checked the shower room window, he found it closed but not secured, to secure it, you have to tighten the screw or the window remains unlocked.
You can't conclude that's the reason unless you know that the particular window catch used for the window concerned is of the type you've mentioned (or similar). Did Sergeant Golding specifically state that the catch concerned had a screw that needed to be tightened to secure the window, as distinct from a catch that just required a quarter or half turn (approximately) to secure the window?
You can easily use something like a hacksaw blade to push the catch out of the slot housing
You stated you believed that Jeremy left via the bathroom window, which wouldn't require dislodging the catch from outside. Do you also believe he entered via the same window and relied on prising it open from outside instead of just leaving it unlocked in advance? If a hacksaw blade had ever been used to open that window's catch from outside, there would be noticeable scratches on the catch caused by the hacksaw blade (or similar instrument).