Update: Customer relations replied today - they issued a new discount code and extended the eligibility period for 1 year (minor glitch is that my code was valid for two passengers on two bookings, the new one is valid for four passengers on one booking). The text of the email seemed to acknowledge that this is a website issue, and indicated it was forwarding it to the web development team. I consider this a satisfactory response within an appropriate time frame (72 hours).