Officially Nissan cannot insist that you get the car serviced at a Nissan main dealer to maintain the warranty. There was a recent government / DTI thing that stated it was counter competitive / monopoly etc. Provided you go to a reputable garage that can prove all the work was done satisfactorily then you should be OK.
As an aside, I recently serviced my car myself and it cost me £77 plus my time. That included oil, filter, plugs, pollen filters, drain washer, brake fluid, coolant, brake service (clean up and regrease), wash and hoover !
My car's out of warranty so it didn't matter that it wasn't done by Nissan and I wasn't prepared to pay Nissan the £340 !!!
for a 72k miles P3 service.