If they are caught travelling short on the advance ticket they will be liable for the single fare from Preston with no refund on the advance fare.
The Conditions of Carriage suggest otherwise:
"If you start, break and resume, or end your journey at an intermediate station
when you are not entitled to do so, you will be liable to pay an excess fare. This excess
fare will be the difference between the price paid for the ticket you hold and the price of
the lowest priced ticket(s) available for immediate travel that would have entitled you to
start, break and resume, or end your journey at that station on the service(s) you have
used."
http://www.nationalrail.co.uk/system/galleries/download/misc/NRCOC.pdf
The ticket conditions say only:
"You may not start, break and resume, or end your journey at any intermediate station except to change to/from connecting trains as shown on the ticket(s) or other valid travel itinerary."
http://www.nationalrail.co.uk/times_fares/advance_conditions.html
So from that, I would say the excess charged should be £53.80 (SOS route Carlisle, the CDS is not available as Preston has a booking office) less the cost of the original advance single.
HHF, may I ask, is there something I have missed somewhere which dissallows this excess?
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These people I know are travelling on the 8th April, on the 1151 Preston-Edinburgh Virgin service. Advance tickets are held through to Edinburgh, but the place where they need to be is closer to Haymarket station. Would they be OK to terminate their journey early at Haymarket, or would they have to buy a ticket from Edinburgh-Haymarket?
By the book, they would have to either go into Edinburgh and back out with an Edinburgh-Haymarket ticket or pay the excess described above, but as stated by the likes of Reb, in practice you may well be allowed the break of journey.