....but on a more general point, what is the actual status of these "vouchers"? If someone lost the voucher, have they broken any conditions of the NRCOC? Surely to stay within the NRCOC, the TOC needs to put up temporary signs at that station saying "you must buy a ticket at this station" and then have a mobile cubicle selling tickets. Why vouchers, not tickets?
If someone lost the voucher, saying something like "they gave me a voucher but I must have lost it", before being told they should have one, could carry a lot of weight.
Mobile ticket cubicles would cost more money than a man handing out vouchers, but the point of the exercise is to catch people attempting to pay less than the correct fare for the journey. Selling a ticket gets you the fare for the day, giving out vouchers allows the TOC to find all the people who only buy what they have to to get through the barriers.
When I worked at City Thameslink they were looking to install ticket barriers and put on a revenue block one day. Ticket sales for the day went through the roof, but money taken worked out at an average of around £1 per ticket. My shift alone had more than 500 extra tickets, but there is no way you are convincing me there were suddenly 500 new travellers going to Elephant and Castle, Blackfriars or London Bridge during that one shift, all of whom obviously decided not to travel again.
The vouchers are little more than a permit to travel (which are well within the NRCoC), but if staff are giving them to everybody and you don't have one, the chances of you having travelled from that station are dramatically reduced.