Hi, this is Jim, thank you for the question - I will resolve this for you.
Sorry to hear of the issue.
You are covered under the Consumer Rights Act 2015 here which means you have a right to expect a service (from the bank) carried out with reasonable care and skill.
You should now make a formal complaint and if that does not resolve the matter ask them for a "deadlock letter", which is a letter giving their final response. Your bank will have a complaints policy on their website.
Once you have the deadlock letter the next step would be to escalate this to the Ombudsman - they will investigate and liaise with the bank which would hopefully result in a refund of the money lost. They can order the bank to make a financial award for inconvenience if they have acted badly (which they have in my view).
You can make the complaint here : www.financial-ombudsman.org.uk/consumers/how-to-complain
Based on the circumstances, assuming the bank does not uphold your complaint, the Ombudsman may do. The Ombudsman will look at this case independently and will make a decision based on what happened. Their decision is binding on the bank, but not you. So you would not be bound in law to follow their decision. If the Ombudsman did not find in your favour, therefore, you then have the option of suing the bank for breach of your consumer rights and you have up to 6 years to bring a claim to the court (the limitation period is 6 years from your date of loss, to when you need to issue a claim).
I hope this helps and answers the question - please feel free to ask me anything else.
Have a good day,