I had a similar issue when I bought my ex-Nissan fleet car, 18 months old and with only a single 80pt check stamp. The car was low mileage (7K) and in great condition, so I asked them to give it a full P2 service before I bought it which brought the car back into date. I carried on with the normal service interval since and have happily claimed on the warranty until it ran out. There's no issues with that at all, as the car has been in their care anyway. If you're that bothered ask them to write a letter stating that the warranty will still be valid despite the lack of stamp, you're covered then.