Here is a table that I've put together that correlates between age, national curriculum level, Oxford reading tree book band and lexile level. It can be used in conjunction with the search tool at Lexile.com to fit any non-reading scheme book into your school's reading scheme instead of spending hours reading the books and trying to see where they fit in.
Simply type in the book that you want to fit into your school reading scheme at the website above. You can enter the ISBN number, the author or title and the website will give you a lexile level. Look up the Lexile level in the table above and it will give you the corresponding Oxford Reading Tree book band,