@Thisonesfortheroad,
Hi, I was reading your comment and if my assumption is right, your child is in Y5 and you are testing the water. He is also in a state school, if I am correct on this, then this is my opinion.
Top tier in a state school doesn't mean anything in top selective schools in this are, sadly said, but a fact. I work in the maintained sector and have a very academic child in the independent sector.
In short, depending on the school, your child needs tutoring. A very small %, and I mean a very small % of children make it in their own steam into these selective schools. Your local state school can always recommend your child for the free tutoring programme at Hampton Boys, if nothing else, it will give your child an idea of what is expected at the Tiffin's boys exam.
We have opted for no tutoring outside school as my DC is a very academic school and my personal approach is that in the right school for him, he will make it on his own steam, but that has been a hard call to make, particularly when pretty much all of his classmates are being tutored for the top schools in England. We are sticking to our position and so far it has been the right approach for us.
Tiffin Boy, you HAVE to tutor for that school from Y3, if your boy is in Y5 it might be too late, but you know how your boy goes. You need to pick a Tiffin tutor to get him up to scratch.
I know a lot of parents are going to disagree with my opinion, but it is just my opinion based on my field of work and my personal experience with a highly academic child in his own right.
Also, check his CATS results, most of the top schools in the area are looking for a 130 as a minimal result and if your school doesn't do one, you can expect his next school to do it as part of his entrance exams.
If your child has failed the KGS 10+ exam, I don't think he will make it through phase 1 at Tiffin unless he starts some serious tutoring right now.
You might want to have a look at some "less academic" independent schools if you have your heart set on the independent sector. Said this, none of our friends coming from top set state schools without tutoring got a placement offered in the schools they applied. BUT it is NOT impossible despite me painting a very dire picture.
ATOM, BOFFA are good online tutoring websites, but you might want to find an online tutor that has the experience on 11+ exams to do the tutoring. Private tutors for these exams are not cheap nor in abundance, but make sure the person has 11+ exam entrance experience as it is a whole different ball game.
Good luck!