I am also currently studying with CATS. I just use the online course and find it more than adequate. If I get stuck on topics, most of the time I can refer back to my PPL manuals to re-grasp the basics. If I am really struggling I just send the instructors an e-mail and they are very quick to respond. As already mentioned the contents of the ATPL course is not overly difficult, there is just a lot of it!