Wouldn't be concerned as others have mentioned, so long as up until now it has decent history. P3 will cost upwards of £250, so knock that off your offer. Talk to the seller, ask if he has done his own servicing as well, if he says no, then you know the complete history, if he says yes, then there's a chance there has been a few extra oil changes.
I know what you mean about keeping the stamps in the book, I'm in the same boat. I actually know one of the mechanics in my local Nissan dealer, who will happily service it on the cheap outside of work, but obviously no stamp.
What's the thoughts on having him sign it anyway instead of a stamp?