Mechanic's mistake but client must pay?

My wife's new Nissan was taken into Nissan Brits for it's scheduled service, when the car was collected there was a chip in the paintwork & one of the carpet mat's was missing. After a fight I eventually got the mat back but had to live with the chip in paintwork. When the car was collected after the second scheduled service the vehicle shuddered during breaking. I spoke to a few motor mechanics who told me it meant there was a problem with the break-disc's, either they were not fitted correctly or dirt had got into them when being fitted. My wife returned the car & was told the break-disc's needed replaced & we were given a price. When I spoke to the Nissan mechanic he told me they had replaced the break-pads at the last service but didn't look at the disc's. I explained what I had learned from the mechanics I had spoken to & very unprofessionally the Nissan mechanic ridiculed the mechanics I had spoken about. I find it very hard to believe the Nissan mechanic replaced the break-pad's without seeing that the break-disc's also needed replaced, I also find it hard to believe that if the disc's did need replaced at that scheduled service, that they weren't. Which makes me think the disk's were not due to be replaced at that millage. So how is it that the car goes in for a service without a shudder, & without needing the disc's being replaced, but comes out of the service with a shudder & with the disc's needing replaced? Strange, isn't it? The bottom line is that the Nissan mechanic replaced the break-pad's & when doing so either got dirt in-between the pad's & disc's, or fitted everything back in place incorrectly which caused the shudder which caused the untimely deterioration of the break-disc's & instead of admitting his mistake now wants to charge my wife for the replacement of the disc's. Two services & two bad experiences. Great ratio! First time Nissan buyer, BUT NEVER AGAIN!!!