I have started (many thanks to foxabout2pop) making enquiries about tutors.
The one I spoke to tutors specifically for Tiffins as the VR & NVR exams are so different to the standard independent school exams (ie English, Maths and a bit of VR). She said that only children in the top sets for literacy & numeracy should apply, and even those who are very bright have less chance of getting in if they have no tutoring in this kind of exam. For one thing, speed is necessary in answering these kind of questions so practice is important.
Apparently these exams were meant to level the playing field and find those with native intelligence & good at problem-solving, but as time has passed it has developed into a subject of its' own. It sounds like even the cleverest child stands no chance if they've never done this kind of thing.
The tutor stressed thinking about what kind of school would suit the child best, not just at 11 but at 16. Both Tiffins are very tough, very academic, although there are lots of other activities as well. It's difficult to judge at Open Days as they are so crowded.
The tutoring is usually 1 session of 1.5 hours a week, with an hour's homework. They normally start in January a year before the exam, although increasingly some are starting in the September (so they would have 4 terms of tutoring). The turor in question recommends booking at the beginning of yr 4.
So it seems that if Tiffin is your main choice it's best to tutor even the brightest children. I forgot to ask about cost, does anyone else know this? I'm still doing research so will report further if anyone is interested.