This is quite an unusual case, assuming the facts are correct (couldn’t buy ticket at station TVM for whatever the reason was), then didn’t have a problem with paying on train (if a person selling tickets had been found), then went to the equivalent of an excess fare point offering to pay.
Many will say pay the penalty and chalk it up to experience.
Personally due to the unusual facts, might be better in court.
The reasons are had 6 people (one adult, 5 children), so potentially have multiple witnesses about machine not selling ticket. Also have same number of potential witnesses to any discussion about buying the ticket on train or at destination.
I don’t know the ages, but some rail companies (and I haven’t checked Northern) have rules about children below certain age needing to be with a responsible adult. I suspect a magistrate would take that as adult is expected to pay, and not the other child passengers (this removes the option of someone else could have paid for the group)
I also suspect if it was adult supervising children a court (and any defending solicitor) would want it as a single case involving 6 people, rather than 6 separate individual cases.
Then Northern would be expected to prove they were trying to avoid paying, and that options like ticket office being open, machine guaranteed as working and not offline to bank; and conductor made themselves available in all carriages during the journey (basically any reasonable impediment to getting the ticket) didn’t apply.
Then there will be evidence on cctv of the group enquiring about paying at Leeds (and only an idiot would overwrite barrier line cctv recordings whilst disputes are open), as I said before if having opened a dispute the cctv evidence is destroyed it won’t look good to a magistrate if it effectively hides proof of the defendants action.
The journey won’t be in dispute, but trying to prove the whole group were fare evading in court, rather than hadn’t been able to get a ticket until told wouldn’t sell them one sounds like a tall order. I guess they could try and remove the witnesses as unreliable due to age, but I suspect a magistrate will see that as desperation rather than a sensible reasonable approach.