Stressedstatemum, I feel your pain - it feels so unfair when kids at private schools get tutored especially since round my way they seem to book up the most sought-after tutors! It's as if tiny classes, no/few very challenging children, cats tests, tests, interview prep, general exam technique etc etc that they get at school isn't enough... I do wonder what they're paying for. The OP says her daughter is at a non-selective prep but surely there are no non-selective preps - my ds had kids in his class who couldn't count to ten by y6 (very disrupted home lives, resistance to any diagnosis).
Anyway, when we did it for my ds, I did wonder how on earth he'd have any chance. Top table but not effortlessly so, very deprived primary in disarray for various reasons. In the exams, he was a polyester fleece in a sea of stripy blazers. We only did three schools and they were all pretty selective (top 30 I think, and bear in mind that it's even worse for boys since some previously boys' schools have gone co-ed). We felt as you do, that he'd have a more able cohort in the top set of a comprehensive than in a less selective school (where we are it's all or nothing - v.selective or not very at all at 11+ - once actually there, there are lots of less able children but they've got in at 3 via the attached prepreps).
While we were doing it, I wished we had put a less selective school into the mix just so he'd get in somewhere, but we didn't because I knew it would be doing an additional exam for no reason (and frankly three exams, three interviews, was bad enough).
Anyway... in the end, he was interviewed at all three and was offered two of them. At one point, we were convinced he wouldn't even get an interview as he seemed to litter his practice papers with careless errors. Now he's there, he's doing well and was ahead in maths (behind in science, languages, but has easily caught up in the former).
It's grim, it really is. Even though it worked out in the end, I do feel a bit scarred. Annoyed at the schools for bragging about at the great results they achieve with a group of highly trained bright kids. They feel like hospitals that only take in healthy patients and then brag about how good their doctors must be since so many less of them die than the NHS place down the road.
On the positive side, at least doing it from a state school you're not surrounded by other parents/children all stressing together. And we know that children can and will do very well in state schools whereas the privately educated primary families I know where in a frenzy about it since they were sure that a comprehensive would be worse than borstal.
Sorry you've got an essay from me, but it really brings it all back, especially since we're stupidly doing it all over again next year with the sibling...