I couldn't agree with you more fm, we're a classic example of exactly that mistake you quoted.
We (being completely new to the whole experience), let our relatively bright DS to sit VR papers (Bucks exam), around 3 times a week (from many of the sources mentioned on this site) consisting of books, papers & tests. Because he enjoyed doing them, I didn't ever sit back & realise we were doing too much. Then a few weeks before his actual exam I started looking at his falling scores & encouraged him then to sit even more
, he of course got worse, so 2 weeks before, I suddenly realised the mistake we'd made & didn't let him look at another VR paper until 2 days before his test, his score still wasn't as high as in the early days but it was a good pass. He did 'just' pass the real exam.
In complete contrast he went on to sit Kent & Medway tests passing with high results, after just a few days training on NVR before the test.