It depends, if they give you a recent past paper then it's a good indication, although you have the whole of the Christmas holidays to prepare the areas that you're weak in.
They generally say that what you get in the mock, you can push the grade up by 10% (a grade boundary) in the real thing.
it depends really. Last year in one of my physics mocks, i got a U, then got an A in the real exam. If you get rubbish in the mock you know you have to studystudystudy over chrimbo!
It's fairly simple. I pissed around all year in lessons. Knew nothing for the mock, and I'm not that good at Maths as it is. Then I got a magic workbook, completed it two weeks before the exam and blasted through past papers, then did the exam. I've got my certificate and I've still got my mock if you really want proof. I'm not really sure why someone would lie about something like this though.