I think you have to do some preparation if there is none at school, they may not have covered the whole curriculum, and they will benefit from having experience of working in timed test conditions, and practising VR questions. I have a DD who was not at a prep but the overseas equivalent of a good state primary school that didn't prepare them at all. We did nothing to prepare until after this half term, then I did some Maths papers with her (they were not Bond but the ones then used at the end of Year 6 to measure ability and attainment, NFR?) and we then worked on anything she didn't know or was weak on (the school hadn't yet covered the entire Year. 6 curriculum). No formal timetable, no more than an hour / 30 mins when we could, we were in a lovely warm country with a communal playground beneath our flat so often I just couldn't bear for her to be stuck calculating the area of a football pitch when she could be out playing. She had about 8 one hour weekly sessions with a favourite teacher to work on her literacy, which was her weakness. And she did lots of logic puzzles and problems which she enjoyed (and still does )
Fortunately I was completely unaware of all the playground Chinese whispers etc that get everyone wound up over here!
They are looking for bright DDs with potential, not attainment and tutoring, and so the maths paper seems to test a basic level of knowledge but then has problems that test familiar concepts but in unfamiliar ways so that they have to apply logic and lateral thinking skills. It is similar to Godolphin and Latymer's and they have sample papers on their website.
The English paper is straightforward. Both DDs are dyslexic, so they had eccentric literacy skills but in both cases comments were made about their lively, imaginative and original stories.
LEH also have a general paper with Verbal Reasoning questions, so worth doing a few practise papers, and general knowledge questions, the general knowledge questions are ones that any reasonably well read DD will be able to answer. Eg on DDs paper there was a question that gave you one out of title and characters from children's classics and they had to provide the other and the books were from memory, Harry Potter, Swallows and Amazons (titty), The Silver Sword, Little Women, and one other contemporary one that they were all reading at the time. Please don't now work your poor DD through Penguins children's classics because I am sure they didn't expect them to know them all but rather it gives them an idea.
They do interview quite a large percentage who show they have something across those papers.
I hope this helps. Also stay away from the playground scaremongering! Quite a few parents in my younger DDs year seemed to think that tutoring was the magic key but I tend to think it is having a bright, interesting and stimulated DD.