The 'Treetops' books are Oxford Reading Tree, aren't they? The National Book Bands document rebanded books from a number of schemes, giving a colour band to each book. (It included all of the Oxford Reading Tree books). Most schools have a copy of this. (I think it;s a thick green A4 sized book). There is also a list of how the Book Bands colours relate to NC levels. If you Google 'Book Bands + National Curriculum levels', I'm sure you'll be able to find it. Of course, I wouldn't just use this to level children- I'd use the APP statements but I suppose it'd give you a atarting point to know which APP statements to look at!