You do not need a Study Text provided that you are watching the free lectures that go with the Course Notes. In the lectures I work through the lectures and explain what is happening and why, and expand on the notes.
If you are not watching the lectures, then yes – in that case you do need to buy a Study Text from one of the approved publishers.
(What you definitely need whether you are watching the lecture or studying from a book, is a Revision Kit from one of the approved publishers. They contain lots of exam standard questions, and it is vital to practice as many questions as possible.)