Hello, I don't know if you've managed to find somebody by now, but I thought I'd answer just in case! I'm afraid I don't know a particular tutor, but can give you some pointers if that would help. Feel free to ignore if you know this already though!
Withington's is a bit of a funny entrance exam, it's designed to test for potential, so it looks at how you tackle the questions, not just if you get the answers right. It's hard to tutor for it as they'll ask questions above the level expected for year 6 to test how the girls think.
There are two English papers (comprehension and essay I think), two maths (paper A is straightforward questions, paper B is more problem solving and applied questions) and a non-verbal reasoning test.
The best thing to do (whether you find a tutor or not) would be to get hold of some of the papers (if you go to the open day they sell them then) so your DD can go through them and get the feel of them. If she's never done non-verbal reasoning before you definitely should have a go as they're hard if you've never seen them before, they're different to anything she'd have seen.
I would say one of the most important things though is to be well read, it makes such an impact on vocab in the essay and it makes the comprehension easier too. It also helps at the interviews, I'm pretty sure reading is one of the things that's often talked about (although maybe not as much with the new headmistress, I'm not sure).
If I'm not being too nosy, is she at a prep school (because then you'd probably know all the above
)? Where else is she thinking of sitting?