Many thanks, into the loft I go !
May need a trip to the loft.
The only way I could find the number was on the box - it's the SSN code (beginning with a letter, most probably C) rather than the CSSN code which is stored on the N7.
And you should qualify as the coupon is a sweetner for those people who bought an N7 before the price was reduced. I bought mine last month and have managed to submit my details, so I guess I have to wait for a confirmation email at this point.