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I've lifted the full conditions regarding refunds from the National Rail website and pasted them below. Bottom line is that if you turn in an annual season where you bought it then you'll receive the difference between what you paid and what you would have paid (using a combination of monthly or weekly tickets as appropriate) less an administration fee.
If your proposed journey is wholly within the Network Railcard area (see
here for details) then an annual season will also be an annual gold card which comes with a range of
perks like cheap off-peak first class upgrades and the ability to purchase discounted tickets for yourself and travelling companions.
Full refund details are here:
Refunds
If you stop travelling and no longer need your Season Ticket, please hand it in at, or post it to, the ticket office where you bought it.
Any refund is calculated from the date the Season Ticket was handed in. It will be the difference between the price you paid and the cost of a ticket or tickets for the period for which you have actually used the ticket, plus an administration charge (see section 9). Because of the discounts on longer term Season Tickets, refunds are not made pro rata to the periods before/after surrender and Annual Season Tickets have no refund value after about 10 1/2 months. For this reason we recommend that employers' Annual Season Ticket loan schemes are set up so that reimbursements are made in 10 equal monthly payments with two 'free' months at the end of the year, rather than in 12 equal monthly payments.
There must be at least seven days remaining on a Monthly Season Ticket or at least three days remaining on a 7 Day Season Ticket to obtain a refund (although on some 7 Day Season Tickets there may be no refund value after three days of use, dependent upon the relevant Standard Day Return price.
In cases of illness, the refund can be backdated if you produce suitable documentary evidence for a period before you hand in your ticket, provided that you have not started travelling again using your Season Ticket since your illness.
We do not give refunds or allow Season Tickets to be extended for periods of non-use, for example holidays. We do not normally give refunds on duplicate Season Tickets although consideration will be given if you can give written evidence of redundancy, prolonged illness or similar circumstances or if the original ticket is recovered and returned to the issuing ticket office within one month of its loss. Please ask at the ticket office for more details.