I don't know what the margin is but if you are unable to buy the correct tickets for your journey a few minutes before the scheduled departure of your intended train, then I would suggest that this is classed as 'having no opportunity to purchase before boarding.' The
TOCs▸ argument is that allowing
TVMs▸ to sell off peak tickets in the peak will lead to people either mistakenly or deliberately buying the cheaper product thus depriving the TOC of revenue. I don't buy that argument. All it takes is a clear message in large type to be flashed up on the screen before the transaction is completed. Something like:
WARNING. Tickets are issued subject to The National Rail Conditions of Carriage and Railway Byelaws. This ticket is not valid for travel on any train scheduled to depart before 0915. If you board a train before this time you may be liable to prosecution for fare evasion.Currently in these circumstances the rail industry wants you to either purchase a 'Permit to Travel' or a ticket that covers part of your journey. The National Rail Conditions of Carriage say:
If you cannot buy an appropriate ticket for the journey you want to make because the range of tickets that is available at the station from which you intend to start your journey is restricted, you must buy a ticket or Permit to Travel before you travel that entitles you to make at least part of the journey. Then you must, as soon as is reasonably practicable, buy an appropriate ticket to complete your journey. In these circumstances, you only need to pay the fare that you would have paid if you had bought a ticket immediately before your journey.
The price you will have to pay will be reduced by the amount paid for the ticket or Permit to Travel.
Whilst this is the official line, in practice it is very difficult to enforce. How does the passenger know whether the price they will be paying for a ticket that covers part of the journey will be less than the correct fare? Also a passenger may be unfamiliar with the route they are taking to their destination and they may not know which station to pick that, 'entitles you to make at least part of the journey.' And I'm fairly certain most guards/conductors/train managers would rather just issue the correct fare than faff around doing an over-distance excess.