What should the structure of a good personal statement be?

Well, that's a kind of tricky question. I'd say that the first and last paragraphs are quite rigid: the first should be a small introduction into why you are fascinated by the course you're applying to while the final paragraph should wrap up all the points you've made in the statement and tie in nicely with your chosen degree. The paragraphs in between depend largely on your chosen course. You'll need to give evidence of any experience you've had, any books you may have read and a little on why you chose your A-Level subjects. In my case, I talked about my work experience at a local engineering firm, explained that I'd read a book called "Why Planes Fly" and mentioned that studying Maths and Physics naturally led me to an Engineering course.