@David Getling: I, too, wonder about the subject knowledge of some science teachers, especially in physics and chemistry. Not infrequently, when tutoring students, I find myself saying to them, "Whoever told you that?" Admittedly, sometimes students misunderstand what they hear in class but the same things keep coming up. Often, student problems stem (no pun intended) from either having difficult concepts poorly explained to, or simply told things that are wrong. Student teachers and NQTs would come to me for advice, as the 'great uncle' of the department, as they could do so without it getting back to their mentors. In the last decade or so of my teaching career, their problems seemed to be more with what to teach, rather then how to teach it. My mind boggled when I found myself explaining GCSE level science to people who had degrees in the subject.