I think issuing the UFN for the full Anytime ticket is a bit cheeky of GWR, given that the encounter happened at the origin station where, had Alex been notified of the irregularity, he could presumably purchased the Super Off Peak ticket from a ticket machine or ticket office counter. From the reference to "National Rail", it sounds like they were referring to section 9.2.2 of the
National Rail Conditions of Travel, however its pretty clear to me that this only applies if you actually managed to board the train:
NRCoT said:
9.2.2. To charge you the full undiscounted anytime single fare to a station directly served by the train that you are on. You will not be entitled to any discounts or special terms, or for a Ticket to a station other than one served by the train that you are on
Regarding the possibility of criminal prosecution, only the more serious
Regulation of Railways Act mentions
attempting to travel without a valid ticket:
Regulation of Railways Act said:
If any person—
(a)Travels or attempts to travel on a railway without having previously paid his fare, and with intent to avoid payment thereof
however to achieve a conviction here, GWR would have to prove intent beyond reasonable doubt.
The less serious
Railway Byelaws only make it a crime to actually board a train without a valid ticket (section 18), unless the station is a Compulsory Ticket Area (section 17), which I doubt Castle Cary is, as they are usually only present at stations in that are in Penalty Fare areas, which tend to be commuter lines with frequent stops and/or Driver-Only Operated trains.
As such, I would be minded to send a cheque for £43.30, which is the cost of the walk-up Super Off-Peak ticket, however you may wish to just pay the sum they are demanding, to avoid stress and hassle. Separately, you can claim a refund of the unused original ticket from whichever company runs the app, minus a £10 admin fee, under section 29 of the
National Rail Conditions of Travel