The Trafford exams are opt-in, which means that, generally, those taking the exams are in with a chance of passing. You do get some local kids who take the exam because they are local and it's the thing you do however bright you are but, increasingly, this is getting rarer.
The mocks take things one stage further because again, generally, it's only those parents of kids who are potential passers who are also genned up enough to know of the existence of the exams and want their kids to have a chance.
I think that, therefore, an average performance in the mocks doesn't equate to an average performance nationally. Unfortunately, it doesn't tell you what it does equate to.
I think the mocks work in terms of exposing your child to an exam-type situation, time management and for finding out which areas they are not as good at (assuming the mock results provide this information). They don't work in terms of telling you that your child will pass the real thing.