The Oxford Met CD is top notch.
I'm doing the Bristol Ground School, their CD is pretty good, but in places I could also have done with extra explanation. I bought the Oxford Instrument book to supplement, but it didn't help. Still passed comfortably. Their CD is pretty good for the most part though.
I think you will find that your revision course will cover the bits that you're not getting so wouldn't worry about it too much. If what you're not getting is a small part of the bigger picture, I'd worry about it later. You may also find that what you're worrying about is not actually examined, just part of the learning objectives. So don't get too caught up in it.