From the conditions of carriage:
10a) General
10a1) We will refund the fare and any surcharge for your ticket, or any unused part of it and any taxes, fees and charges, as set out in our fare rules and tariffs.
10a2) Unless we say otherwise, we will only make a refund to the person who paid for the ticket.
10a3) You must, if you want a refund, prove to us that you are the person who paid for the ticket.
10a4) Unless you are applying for a refund on a lost ticket, we will only make the refund if you first give us the ticket and all unused flight coupons.
10b) Involuntary fare refunds
10b1) We will pay fare refunds as set out below if we:
cancel a flight
make a significant change to a flight time which is not acceptable to you
delay a flight by five hours or more
fail to stop at your place of destination or stopover
cause you to miss a connecting flight on which you held a confirmed reservation or
refuse to carry you because a banning notice is in force against you.
10b2) If you have not used any part of the ticket, the refund will be equal to the fare and any surcharge and any taxes, fees and charges you have paid.
10b3) If you have used part of the ticket, the refund will be equal to at least the difference between the fare and any surcharge, taxes, fees and charges you have paid and the correct fare, surcharge, taxes, fees and charges for travel between the points for which you have used your ticket.
10c) Voluntary fare refunds
10c1) If you are entitled to a refund of the fare for your ticket for reasons other than those set out in clause 10b, the refund will be as follows.
10c2) If you have not used any part of the ticket, the refund will be equal to the fare and any surcharge and any taxes, fees and charges you have paid, less any cancellation and reasonable service charge.
10c3) If you have used part of the ticket, the refund will be equal to the difference between the fare and any surcharge, taxes, fees and charges you have paid and the correct fare, surcharge, taxes, fees and charges for travel between the points for which you have used the ticket, less any cancellation and reasonable service charge.
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10e) Our right to refuse a fare refund
10e1) We may decide to refuse to give you a refund if you apply for it after the end of the ticket validity period.
The ticket validity period is also defined in the c of c's:
3b) Ticket validity period
3b1) Unless it says differently on the ticket, in these conditions of carriage, or in any tariffs which apply, a ticket is valid for travel for:
one year from the date it is issued or
one year from the date you first travelled using the ticket, as long as your first flight took place within a year of the ticket being issued.