@davidmanch, the sending bank is liable to compensate you. Regulation 91 of the Payment Services Regulations 2017 (which is the UK’s enactment of Directive (EU) 2015/2366) states:
(1) This regulation applies where a payment order is initiated directly by the payer.
(2) The payer’s payment service provider is liable to the payer for the correct execution of the payment transaction unless it can prove to the payer and, where relevant, to the payee’s payment service provider, that the payee’s payment service provider received the amount of the payment transaction in accordance with regulation 86(1) to (3) (payment transactions to a payment account).
(3) Where the payer’s payment service provider is liable under paragraph (2), it must without undue delay refund to the payer the amount of the non-executed or defective payment transaction and, where applicable, restore the debited payment account to the state in which it would have been had the defective payment transaction not taken place.
(8) Regardless of liability under this regulation, the payer’s payment service provider must, on request by the payer, immediately and without charge—
(a) make efforts to trace any non-executed or defectively executed payment transaction; and
(b) notify the payer of the outcome.
If the payment was sent before this legislation was enacted, then it might be subject to very similar provisions under Section 75 of the Payment Services Regulations 2009.
Has the sending bank proven that the receiving bank received the payment? If not, and if the FOS are dragging their heels, you should instruct your solicitor to issue a County Court claim against the sending bank. As the amount is above £10,000, it won’t be in the Small Claims track, and you can recover your legal costs from the losing party.