Doesn't work like that mate. You get back what the letting agent decides, if you want to challenge it you need to go through a arbitration org. You may get it all back but it's not easy!
I've just paid £300 for two of us, I hate the bastards. If a credit check with Experian costs IRO £5 there is no reason why they should charge that much.
I'm also resigned to the fact that no matter how much I take care of my new flat, that when I leave I will not recoup 100% of my deposit!