A few things to bear in mind.
1) Since the beginning of April, Northern's "Customer Experience Centre" has been run by Carillion on Northern's behalf, in a building that no Northern staff go in, employing people who have probably never worked in a railway enviroment before and seem to have very little railway training. The result of this is that I think it very unlikely that anyone from Northern would actually see your complaints unless escalated beyond the "Customer Experience Centre".
2) Guards across the country, not just in Northernland, have to deal with people attempting to use tickets more than once. This does not excuse cases of staff getting things wrong, but does sometimes explain why guards are more suspicious of tickets being used over multiple days. I'm sure someone can suggest a bulletproof plan to combat this, but in the meantime it remains a problem.
3) The OP may not have gotten the £6.55 owed in a refund, but they did apparently get £10 in vouchers which could be used for their next trip. I realise it should have been a refund by the original payment method, but some might actually consider this better than the refund. The Op could contact Northern Carillion again for the refund if they so wish, but I make no guarantee about the outcome.
4) I noticed someone suggesting, if the guard believed the ticket was not valid and an Anytime ticket would have been, that an excess to the Anytime fare should have been offered. The Op was sold a £6.55 ticket, an excess fare would have been nearly £28. I'm not suggesting the guard was completely right in his actions, but simply that, had they been correct to charge more, an excess fare was not appropriate.
5) Re-briefing these days generally consists of sending out an email with an attachment to print off (but don't print it off because we need to save the planet), putting a piece of Northern branded A4 paper on a notice board, or hoping staff gain the ability to read minds.
