I don't think that is correct. According to the FCA website (
https://www.financial-ombudsman.org.uk/consumer/complaints.htm) Well, you are unhappy with their response - they haven't bothered.
When I had problems with an energy company who wouldn't consider my complaint (send by e-mail - evidence of receipt), just offered a token payment, I gave them a stated time to reply, which they didn't. Off to the regulator, sent all the evidence - again, e-mail. Compensation four times the original offer. Different regulator, I know but I have got dosh from the FCA (when I didn't ask for any, I just wanted a written answer to a technical question when the bank insisted on a phone answer as they hadn't got time to answer written questions; I also got a written answer).
Maybe too late in this case, but I would encourage you to push it, if you feel aggrieved. In my opinion the vast majority of bank accounts are with banks who are at best indifferent to their personal customers most of the time.