"Most people who are not middle class are still 'educated' enough to help a child at KS2 level, if they choose to do so. "
Really? On what basis do you say that? Where is the proof that the millions of people who have left school without a single qualification would be able to tutor their children through 11+? That parents who do not have any books in their house and do not read for pleasure suddenly have no problem in helping their children in verbal reasoning and comprehension?
My parents couldn't tell you what "non-verbal reasoning" is.
"For the parental help argument to be justifiable there has to be evidence that the correlation between 11 plus performance and socio-economic background is greater than the correlation between socio-economic background and performance in other school exams in state schools in non-grammar regions."
Are you talking about KS2 SATS here? Because of course if you are looking at GCSE etc then you have to factor in the advantage of having a selective school in the first place. So I guess the broad stat that would help you would be % of children on free school meals who get L5 at KS2 versus the % on FSM who get into grammmar schools?
But on the other hand this article from earlier this year seems to suggest that many of the private school pupils who end up at grammar schools have lower SATS results than their state educated peers. Presumably they will have pipped somebody else to the place?
Anyway gone well off topic. Have every sympathy to the OP who is probably in the final throes of trying to prepare their dc for 11+. The pressure of just one day seems v crazy. Not looking forward to it!