Hi I'm really new to driving so just need some advice.

Someone went into the front of my car 2 weeks back, the repair garage has picked it up (they drove it there) it was driving fine when I was using it. An hour later they called me and said the guy has just come back but your clutch has gone and they worked out how much it's going to cost me, but do I just take their word for it? The only thing is I agreed to it so I know I will probably have to pay it but is there absolutely anything I can do or not? Some people have told me they could be making it up etc, I'm so stuck on what rights I have, if there is any?