So you'll know that they are obliged to sell a fare that is both 'permanent' and 'basic'.
I'm guessing this fare does not meet the criteria, as the National Rail website states:
http://www.nationalrail.co.uk/times_fares/promotions/prcb15272552460da45fc2d27756cdbe/details.html
National Rail Enquiries said:
How to buy
Tickets can be purchased from:
- Bus drivers
- Metro Ticket Machines
In which case, the ticket office staff are not obliged to sell it.
Are you sure the staff were in breach of the rules on impartial retailing, as defined in the Retail Standards Guide?
Also, as AlterEgo says, if you ask for a ticket by name then they can sell it without question. e.g. if I say "Off Peak Day Return to Leeds please" the most I could be asked is if I wanted to go via Harrogate or not. If I said "On the fastest train" then they wouldn't even need to tell me about the Route Harrogate option.
If your complaint is that the ticket should be sold at railway stations, then I'll agree with you, go and contact Nexus, your MP, Northern, ATOC, your local council and anyone else who will listen.
But if your complaint is that staff acted incorrectly, then unfortunately it's not a legitimate complaint.