National Rail Conditions of Travel - Part F Your Refund and Compensation Rights

31.1 If you have a first class Ticket and the train service you use is shown as offering first class accommodation at www.nationalrail.co.uk, but when you travel first class accommodation is not provided or is otherwise fully occupied, you may claim a refund. The minimum refund to which you will be entitled will be the difference between the price of the first class Ticket purchased and the cheapest valid standard class walk-up fare available on the service you used. You must contact the Train Company you travelled with to notify them of your claim within 28 days of the date that you travelled.

31.2 If you hold a first class Season Ticket, the minimum to which you will be entitled will be a refund of the difference between the first class and standard class fare for the journey concerned.