Hi Jade,
I have used carhire3000 4 times so far without any issues. I am using them again this year, I have been unable to find anyone that can beat their price. They answer any queries very quickly via email, and even when i bumped our car last time, it was no problem the insurance covered the whole thing.
One tip, get a quote on the website and chose to email the quote to yourself, after a week or so you may get offerred a discount on your quote to try and get you to make your booking. Our quote went down by 30 pounds just because we hadn't confirmed it.
They use alamo, national and dollar. And if you want one of these in particular they can usually confirm the company for you if there is a car available, so you can be sure of which company you get. I have had alamo mostly and dollar last time, and no problems with either.