Max just had a similar issue with the local dealer although his car wasnt under warranty. The mechanic sat in the car, started the engine and said it needed a new gearbox. One thing that has become very apparent to me recently is that the sign over the garage is no reason to assume they have any knowledge of the car.
Without getting into the specifics of this problem I would suggest contacting Nissan Custome Services. I have found that whilst they may not react very quickly they will ensure that the dealer is playing the game. Its difficult to get them to use e-mail rather than phone calls but I prefer to have things in writing.