We did not tutor for the 8+, but I got the impression that the majority of kids in my son’s class had tutoring, either an out of school arrangement or extra paid hours from one of the school’s own teachers. And this was on top of all the 8+ prep that the school already did in normal hours and after school clubs! For some kids this was more than two years of tutoring, starting in Y1 at the age of five, having a go at 7+ and then another go at 8+. The irony was that it seemed to make no difference to whether kids got into the highly sought after schools (St Paul’s Juniors etc). The early developers who were reading CS Lewis and using perfectly regular joined up writing aged six got in, as you would expect, and would have done regardless of tutors, and the rest did not, and no amount of tutoring would have got them in.
Having said that, I would use a tutor in future if a) my son was trying for some kind of exam for which his school didn’t prepare, either curriculum wise or format-wise OR b) If he was really struggling in a particular subject and the school was unable to help.