VT are not allowed to make up their own rules that less favourable to the passenger.
Currently on a London bound VT service. The TM has just announced that holders off off-peak tickets will need to purchase a new full priced ticket.
The guidance notes that this should be an excess which is the difference between the fare paid and the appropriate fare.
It goes on to say how there is no penalty even if there was an opportunity to buy before boarding train.
The difference between the correct process and the TMs suggestion would be at least £ 80!
Another example of VT making rules up which are more favourable to VT?