Nice post, Graham.
To the OP, I used CATS, they were fine. There were a couple of Spanish guys on my brush-up course, plus Italians, Germans, French, South African and more... The 3 chunk setup worked well for me, made each exam period that little bit less stressful. I got first time passes on all the subjects, so no wasted time.
I've visited Oxford, and the earlier reference to Bristol QB (ATPLOnline) is wrong only in that it isn't on the quiet, the use of this resource was quite open. Fact is I think everyone uses them. But I wouldn't advise reliance on this, whichever school you use will have their own in-house version, I'm sure.
I can't comment on Bristol's GS material since I haven't seen any. I did borrow a couple of Oxford CDs, which are great for a) putting you to sleep - the narrator is quite monotone; and b) if you can stay awake, explaining stuff quite well.
I get the feeling that wherever you go, if you put the work in, you'll get your passes so no point losing sleep. Just go with any that suits you.