Oh and OAA's CBT system is a complete joke, not only does it constantly crash, the questions in their bank are often wrong and certainly do not represent the actual exams, I think after 14 atpl exams I maybe saw one or two from oxcell, compare that to 90% of questions in exams being very similar or identical to Bristol's it's a no brainer.
Also some of the core subject material on the CBT system is wrong, even some of the instructors advised students to stay away from it as it is very confusing, radio nav and instruments were particularly poor. I would say that most of the subjects had errors.
If I were to do it all again I would definatley use Bristol, if a friend asked for advice on how to approach ground school, I would recommend Bristol.