As said, first one should be less than £200, there could be software updates to do, navigation might need an update, keep it with the main dealer when in warranty, Our Navara at over 4 needed a aircon condenser, no charge, another Navara almost 5 ,big mileage had a new engine, again under warranty.
Sigh....they've got to you too. 5 year warranty on new Nissans, right? And you're pleased that they've done warranty work under warranty?
A dealer/manufacturer can't wriggle out of their warranty obligations just because they haven't been the ones servicing the vehicle. As long as there's a paper trail showing that the servicing has been carried out as per the schedule, with proper fluids and proper parts, they haven't got a leg to stand on if they try to fight it. If they do try to fight it, tell them to go sit on a 4" nail. Any court in the land will tell them the same thing.
I worked in independent workshops for years, never had any issues with warranties in my time. To be honest, I heard of more warranty related horror stories from new customers who were mentally scarred from their experiences with the dealer workshop.