I can only speak from my experience. I have seen another post of yours and I don't think you're in Trafford (you mentioned your DC sat Walsall) - I think the system is very different here. Certainly for me, there are one or two companies - the likes of Kumon etc - but here, there are no "tutoring companies" as such. Most of the tutors (as far as I'm aware) are individuals - ex or current teachers in the main who have set themselves up as tutors.
I think it very much depends on the child as to what makes a difference. I've had 2 go through it (used the same tutor for both). One child would have passed I think (with the benefit of hindsight) regardless of the tutor, she was top of the class but had a competitive, conscientious personality so loved doing homework, reading etc because she thought hard work = desired result.
Other DC - top half of class but not right at the top. Lacked concentration, was bright but wasn't conscientious at all. And lacked confidence. Results were hit and miss. Tutor made all the difference - not in terms of preparation necessarily, but giving him such a confidence boost that he went into the exams thinking he could do it and if there were questions that were tricky, he just had to take a breath and read it again. The tutor was fun, lots of rhymes and little games, DC loved going so it wasn't ever a chore for them. I would say finding a tutor who "gets" your child is key.