My dealer won't repair an intermittent fault under warranty, how do I proceed?

I took out an extra 3 year warranty, paid in full, when I purchased a Ford C-Max. I have since had an intermittent fault of the clutch pedal not fully returning. The dealer has told me that the gearbox will have to be removed to access the brake master cylinder and only if this is shown to be faulty will it be covered by the warranty. As this is an intermittent fault it cannot be easily replicated. We are now at an impasse and I'm not sure how to proceed to get the car fixed because they say if they remove the gearbox and they do not accept the fault I still have to pay. Any advice on what to do next would be gratefully appreciated.
Dual Mass Flywheel. Inside the bellhousing. Absorbs some of the torque between the engine and the clutch and transmission.
Answered by Honest John on

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