So I left it with the dealer's service department this morning.
They've come back and just said "clutch" and quoted ~£450 for the part and almost £900 including labour.
Hopefully it should be covered under warranty, but that still sounds like a lot.
Thing is, it's actually a Citroen dealership, so I'm not sure if they are a particularly good option. I guess if it doesn't cost me anything, it's fine, but I can't help but think I'd be better off going to an independent with more Nissan experience who might find a more specific fault and will probably do a better job?
Might be worrying over nothing, but can't help thinking that all this money is gonna get spent, only for it to have been wrongly diagnosed...