There again, is it fair to assume that a notice in the booking office window telling you to buy on the train overides the notice advising you to buy before you board ?
Agreed.
Though if the passenger was paying by credit/debit card, then it would be possible to use the machine, assuming the machine was in working order, so if the passenger tried to pay by card then the guard may not be happy about it.
If the passenger was intending to pay by cash then of course they have to board the train without pre-purchasing as the Northern ticket machines at stations such as Swinton do not accept cash.
... The revenue protection officer who questioned me told me that it is my responsibility to find the train manager to purchase a ticket onboard the train. This was the first time I had heard of this as I am a Canadian citizen who has only recently settled in the UK....
This is not true, and I apologise that you've suffered bad treatment here in the UK.
Unfortunately EMT do employ people who will treat people badly and give out incorrect information. I know, I've seen it myself and heard many accounts from other passengers of this.
There is no such requirement to seek the guard, though if the guard does come round checking tickets then of course you should ask for the ticket then, and if this does not happen then you should locate the ticket office at your destination (if there is one, which in the case of Sheffield, there is). Nothing more than that.
Sadly here in the UK we don't have an
effective rail Regulator, and we don't have a
fully impartial, effective,
knowledgeable and proactive passenger watchdog either, otherwise these sort of problems would be nipped in the bud and stamped out, and threads like this would be a rarity. But it's not the case, and sadly you're not by any means the first to be falsely accused, and you won't be the last either.
But I am confident that, assuming what you say is all true, and I don't have any reason to doubt it, nothing needs to be paid by you other than the £3.50 due.
I am happy to proof read a letter for you.
Edit: In fact, I've drafted a letter for you (based on your original). You have a PM.