Thanks for the replies and advice. I just noticed I put the wrong airport in the subject, it should say Overnight stay at Haneda Airport
I have since found out that despite sleeping in the airport not being allowed at Haneda you can get away with it. I am liking the sound of xilr8's idea though. Would be good to try out a bit of Tokyo's nightlife while I am there. I think the decision has been made.
Cheers all