My wife's Cinq Sporting has a prob with the petrol cap. The original, lockable cap became almost impossible to undo - I was afraid of breaking the key in the lock, and even when it was off the car the lock was very, very stiff to operate. So I went to the local Fiat dealer and bought a non-locking replacement, and this is also too stiff to operate without breaking the plastic. Is there a known problem with the tank filler neck that the cap locates in? Is there a quick fix? I'm currently using a cheap plastic "one size fits all" job, bit it doesn't seal too well. Help!