Had a look on gov.uk yesterday and found a copy of the franchise agreement and found this:
Section 65.1 states:
"with effect from the Fares Setting Round commencing in January 2017 introduce: (i) a Peak Carnet Fare; and (ii) an Off-Peak Carnet Fare, which will provide discounts of at least 16% on the equivalent ‘Standard Anytime Day Return’ and ‘Off-Peak Day Return’ Fares"
Pleased to discover my memory isn’t completely shot!
No real point in raising this with Northern in the current circumstances but I’ve emailed the DfT asking what (if anything) they’ve done to hold Northern to account for their failure to adhere to this part of the agreement