No I don't think they don't find out in advance. When you filled out the application you would have ticked a box to say whether you wanted the primary school to be informed or not, so they will only know if you ticked/unticked the relevant box (can't remember which way round it was). I should imagine they find out on the Monday - last year my email came through just after midnight on the Sunday, going through it again this year so hoping it's as quick! Good luck!
I think you are confusing the 11+ results - you tick a box on the CSSE application to indicate that they should not let your primary school know your child's result?
The secondary school is allocated by the Local Authority. Our youngest is in year 7 so our Southend info booklet is a year out of date - I can't see anything about whether or when in advance the primary school is told, but this seems to be the case in other areas. We didn't have any option on the online application of saying that the primary school shouldn't be told as far as I can recall but I wouldn't have seen any reason to have the info withheld anyway.