When I did my ATPL (H) exams I stuck with the course material/tests until the week before the exams. Then I started on the Bristol QB taking the practice tests in the same order as I was due to sit the exams. In combination with the consolidation week really, so some of the oddities in the questions could be explained.
Once you have learnt the material you need to know, the Bristol QB helps you learn the way the CAA actually ask/phrase the questions - which can be a little odd at times, that's where it's value was of benefit to me. I found the Bristol questions easier than the School questions, which given they are supposed to be indicative of the actual exams (perhaps not so since the new syllabus has come in recently ?) was a good confidence booster.
What ever you do, don't substitute trying to learn the Bristol QB as an alternative to learning, and understanding, the coursework.