Norvil also sells (certification) plates. www.norvilmotorcycle.co.uk. Look under frame on their catalog alpha list. You must send them a copy of your registration paperwork and they will stamp the plate with the VIN #. they have five different plates with photos of them. The plate and stamping runs about 12 pounds plus postage.
You don't need to go thru the send registration stuff in for stamped new plate. I went to hardware store and bought the right number/letter size metal hand stamp set and did it myself using of course, the original plate for size of numbers/letters. Took 5 minutes and it was done, no big deal.
Blank plates (and the rivets) are available from most Norton parts dealers including Old Britts. There are different styles for different years, so be sure to provide them with the year of your bike. Number/letter stamps are available from most tool suppliers or you can probably borrow a set from your local machine shop.