Not necessarily watertight.
My argument is that the T&Cs cover only refund requests made during 'the season'. This isn't defined by the Club but is defined by the EFL of which the Club is a member. Under the Consumer Rights Act sections 68 and 69 where the T&Cs are not transparent (and potentially open to interpretation) then the act should fall in favour of the consumer. On this basis, the Club has not clearly defined when refunds are or are not allowed, and this can gives rise to justification for refund. This is on top of other arguments under other CRA sections that refer to the bought product or service no longer being representative of what was originally sold.
Furthermore, the Club already has precedence for issuing refunds for next season: someone on here showed their email confirming the Club refunding them in April.
Combined, even if the Club refuse a refund, if gives strong ammunition for a refund from the bank.
This approach is based on legal advice which my family obtained, and has since shared with the Trust - including templates.
If you want a refund, don't just accept the Clubs view, utilise what consumer protection we as 'customers' have and get your money back!