I gave up trying to transfer the credits into a bank account and instead investigated the Gift Voucher option – which seems to be straightforward, glitch-free and easy to use.
The gift vouchers can be used at about 50 mainstream stores and businesses (Asda, Argos, M&S, The Body Shop, Ticketmaster, Deliveroo and so on).
I've experimented with a small amount – £30 – and, all being well, the voucher will be e-mailed to the girl on 12 September.
Others who have struggled with transferring AW credits may find this method worth considering.