Prior to stopping you he would have carried out a pnc check using your registration. If he had entered the wrong details then it would have shown up as something completely different, so would have asked a lot of questions there, therefore I would assume that his book will have the correct details
Back in 1981 my mate and I got stopped on his bike. He had bought his bike brand new, but the shop had provided the wrong number on the reg plate, so when we got stopped it came up as a volkswagen van (coincidentally, it was a stolen van) and me and my mate got detained back at the nick until the whole thing was settled.