Well, I know the feeling Terry, but that's another story.
The damage, if any, will be to the NAP300 Power Supply. Since that's got to go off to Salisbury to be changed from US to UK power, if you've buggered it up at all they are going to figure this out and fix it for you. Hopefully, the protection circuits will have done their work; if there were no loud bands or smell of burning, you should sleep easy. Chances are they will have a quiet word with you along the lines of "You haven't by any chance plugged this into a UK socket, have you?"
Whatever the outcome of that conversation, you're going to end up with a UK spec power supply.
If it were me, I'd 'fess up and send along the NAP300 at the same time for a checkup and if anywhere near due-time, a service. Then I'd know I'm bang up to date (no pun intended) with the best a man can get when it comes to NAP300's.
I'll pray for your NAP300 (and you of course). It's never worked before with amplifiers, but always worth a try, eh?