Update. Received a placatory email from Northern in response to my asking for clarification of policy, inclusive of the following:
"While I am sorry to hear of the experience that you have had, under the Railway byelaws it is a requirement to display a valid ticket when asked by a member of the railway. As you were unable to do so at the time the conductor was correct to request payment of another ticket. The onus is on the customer to ensure that they have everything required for travel when making a journey on the railway."
The upshot seems to be that unlike other TOCs, if travelling with Northern I have to buy another ticket in the event of ticket system failure (this being cheaper than a smartphone or printer, neither of which I own, to otherwise prove purchase). Further, it apparently doesn't matter what station staff advise.
I think buying in advance of travel if my journey is with Northern is not worth the hassle, and I will seek alternatives where possible.
