I think you'll find that using a staff "friends and family" ticket constitutes fraud, especially now you have been made aware that the ticket has been incorrectly issued. You don't know whether you will encounter a suspicious RPI or traincrew.
Whilst obviously you have been missold a ticket, it doesn't mean you can use it!
I find that a good description of fraud is "deliberately deceptive to gain financial advantage".
I do not see how the op currently fits that description.
In any case, this just won't get noticed, Northern Guards will accept it as if that was the ticket that was supposed to be issued in the first place. The Northern Friends and Family ticket would never be used by staff (we have our staff passes) and can be used by anyone fortunate enough to get their hands on a voucher (or in this case, fortunate enough to find a clerk who doesn't really know what they are doing), no ID is required.
Indeed, although Virgin staff issuing a Northern only product - every possibility they'd never issued one before. I would suggest the OP takes the incorrect ticket to a Northern station and have it exchanged for the correct one.
Naff all a Northern station can do.
Can't issue a Promotional ticket without the vouchers that the op no longer has, can't give a refund because Virgin issued it and can't do an excess on a rover ticket. They are just going to tell the op to go back to the station it was bought at.