Hi there, thanks for the question.
If the pedal is sticking at the floor and the master cylinder has been replaced, the pedal has good pressure, and the pedal itself has been inspected, then it's almost certainly related to the slave cylinder or the pressure plate.
To access the slave cylinder on this model the transmission has to be removed, same as for the pressure plate inspection.
How can I get the garage to look into this?
They are adamant that they have checked the car and found nothing wrong. Cars down breakdown for no reason and the rac don't tow for no reason yet they are saying the clutch pedal can not go down and then later on come back up?
I'm having issues getting them and Nissan itself to help - any ideas that will not involve invalidating the warranty?
Well unfortunately without an active symptom that they can work with then your at their mercy.
You could use a smart phone to take a video or picture next time it happens, but who wants to be stranded again? not me, that's for sure.
So you best bet is to call 1-800-Nissan1 and open a case. The Nissan Corporate case manager will work with your dealership service manager to work on a reasonable resolution.