I agree that we are getting confused between facts and opinion. Let's remind ourselves of the facts, which are known to be as follows:
1. The OP purchased an off-peak ticket at Manchester Piccadilly for travel between Manchester Piccadilly and Levenshulme.
2. This ticket was purchased from a machine immediately prior to travel.
3. The ticket had a railcard discount applied and the OP had the appropriate corresponding railcard with them. The railcard was in date and valid.
4. Having purchased their ticket for immediate travel, the OP immediately undertook their journey between Manchester Piccadilly and Levenshulme, taking the next available train.
5. Although the OP purchased their off-peak ticket for immediate travel, their ticket was in fact not valid as off-peak restrictions had just come into force. The valid ticket for the would have been an anytime one.
6. The OP was stopped by an inspector at Levenshulme and made aware that they should have held an anytime ticket rather than an off-peak one. Rather than following the terms / procedures laid out in the National Rail Conditions of Carriage the inspector opted to take the OP's details, rather than selling the appropriate excess fare.
7. During the conversation with the inspector, the OP inadvertently gave an out of date address. The OP immediately realized this and immediately corrected the error with the inspector (within 60 seconds by the OP's account).
As for the speculation that as the OP used an off-peak ticket at a time when they should have used an anytime ticket then the OP has committed an offence under section 5(3)(a) of the Regulation of Railways Act: such an allegation can only be made should there be evidence of the OP's intent to avoid the appropriate anytime fare. Such evidence would, of course, be based on the OP's actions rather than their thoughts. In terms of their actions, the OP purchased their ticket just before they travelled, and indeed having purchased the ticket took the next available train. It seems entirely reasonable that if one asks for a ticket for immediate travel on the next available train (whether sold by a human being or a ticket machine) that they are sold the appropriate ticket for that immediate travel. Given this, I, at least, remain rather unclear how the OP's actions here demonstrate intent to avoid the appropriate fare. The OP may not have paid the appropriate fare, but their actions as reported certainly don't demonstrate intent to avoid it.
Some might also speculate that the OP intentionally avoided an opportunity to correct the wrong fare once they were alerted to the fact it was wrong. Again intent is determined by actions. For any intent to be proven the train company would have to prove the OP neglected the opportunity to correct the situation by purchasing the correct fare. The opportunity for this would have been the sale of an excess fare by the inspector at Levenshulme, an opportunity which was not offered.
There is also some speculation as the OP initially gave an out of date address before immediately correcting that mistake they are liable for prosecution under section 5(3)(c) of the Regulation of Railways Act. Perhaps. But I remain unclear how successful such a prosecution will be given the OP immediately identified the error and immediately corrected it with the inspector. Indeed, the OP's actions of immediately realizing and actively correcting their error (within the space of 60 seconds) seems, to me at least, to be evidence of intent to ensure the correct address was given, rather than an inadvertently false one.
To the OP: these threads quickly move from constructive advice to speculation as posters debate the finer (or maybe not so fine) points of railway legislation and its moral foundation. You should concern yourself only with resolving the matter. Unfortunately there's really not much you can do until Northern receive the inspector's report and they invite you to respond. When that time comes we will be more than happy to assist writing a reply.
In the meantime please do not panic and stress out. The facts of your case are quite clear so hopefully there should be no trouble with the appropriate resolution (which in this case should be payment of the excess fare between the off-peak and anytime fares - nothing more nothing less) being identified.
In the meantime it might be best to hold off any further discussion until the OP receives notification from Northern and needs to seek constructive advice in putting together a response.
