This has been happening with Ford because the clutch slave cylinder failed. When the parts were returned to the customers the slave cylinders were missing on at least two occasions. When a slave cylinder fails, the effect can be the same on the clutch plates as the driver slipping or otherwise abusing the clutch so not returning the parts gives the dealer the opportunity to blame the driver. So I'd take your case to small claims alleging that the clutch failed as a result of the clutch slave cylinder failing and that the dealer has deliberately lost the 'evidence'. You may bemuse the judge. You may win. You may lose. Or, prior to the case going to court, the dealer may even throw in the towel. However, this is a 7 seat Touran. If it has been towing or running with a roof box, the dealer could counterclaim that it had been overloaded and that's why the clutch failed.