How much should I claim against mechanic?

Hi all, I’m preparing a case for the small claims court and wanted to know if it’s worth claiming the amount I plan to.

Essentially last year, my car started making a squeaking noise and I paid 50 pounds for a diagnosis from a mobile mechanic. He said there was no issue at all with my car.

Two weeks later I nearly got into a car accident as my back brakes completely went, and I managed to drive to a local garage. I had to spend 500 pounds on completely rebuilding the back brakes. This wasn’t picked up in my original diagnosis at all.

My claim is for a total of £550 (not including court fees). I don’t expect all of the £500 to be awarded but I would like a proportion as arguably if I fixed the brakes when they were in better condition two weeks prior the bill wouldn’t have been so high. I also wouldn’t have gone through the stress of nearly getting into a car accident.

My question therefore is do I have a case for the £500 claim? Like I said I don’t expect the whole amount back but I was told by Citizens Advice that I should put the whole amount I paid down and the court can decide the proportion. I’m based in the U.K. FYI