My head is spinning with this thread! Jack99, not quite clear as to whether your school is grammar or another type of selective school - but bear in mind that the people telling you the test is 'impossible' to prepare for probably see you child as potential competition for theirs.
I would strongly urge you to get hold of some example papers by hook or by crook. If it is a garmmar, www.elevenplusforum.co.uk is a source of very useful info - and there's a forum for each of the different counties.
You don't have to get embroiled in the rights and wrongs of the whole issue, but you can ask about anything related to tests, scores, standardisation etc. and someone will always answer.
Fwiw, DD has just passed for our local grammar, and I would say she was tutored (so hang me!) as myself and DH went through papers with her and practised the different types of questions.
However, from what she said about the test, I would say that they really are designed to sift out those with the most potential. In practise she always completed even the hard papers in time, and scored at least 85% - but in the test she left out 12 questions(!) and said some of it was 'really hard.'
She also passed for one of the country's top mixed grammars but is going to our local one.
I think the schools are very wise to tutored applicants, and set their tests accordingly.