I just saw someone had found the blog on the search term “bill plant pass rate “.
If you want to be pedantic – and there are a fair number of ADIs out there whose only identifiable skill is pedantry – you could argue that if an ADI is paying a lot of money for his franchise, he might not be making much profit, so he’ll have to cut back on fuel or try and fit more lessons in, so the standard of his teaching will be affected. But then, there are plenty of smaller franchises desperately cutting prices to try and win business, and that means their franchisees aren’t earning much, either.
The bottom line is that if you get a good ADI who teaches you well, passing your test is down to you and not the ADI.
EDIT 3/2/2012: Look at the official pass rate figures in the Information section – these cover a recent 12 month period.