2011 Fiat Other MOT Failure

Stoneacre failed my MOT due to my exhaust being unsecure at the felx and the clamp seems loose. They say I must use a fiat part to replace it and will cost me £387. Most people I have spoken to say it shouldn't have been a failure and even if it is I could either tighten it or spend much less than they quoted replacing it. I think they are taking advantage of me being a young girl. They are a Fiat dealership so I think that is why they told me I can only use a Fiat part to fix it. Can you tell me if this is tru and should my MOT have failed?