Sorry to hear of the issue.
There is an option and it's the only one available in my view, short of pursuing them with court action.
Given the dispute with the bank you can avail of the Financial Ombudsman Service dispute resolution scheme - which is much quicker than court action and free to use.
You have to go through a complaints procedure first.
You have protection under the Consumer Rights Act 2015 here which means you have a right to expect a service carried out with reasonable care and skill.
Furthermore, the account service should be “fit for purpose” and “as described”. If it isn’t then you can allege breach of your consumer rights and breach of contract.
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.
You can make the complaint here: https://www.natwest.com/support-centre/how-to-complain.html
When you do make the complaint, tell Natwest that you plan to report them to the Financial Conduct Authority (FCA) who regulates them. You can reach the FCA to report them on 0800(###) ###-####if you wish. It may speed matters up.
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 resolution of the complaint in your favour. They can order the bank to make a financial award for inconvenience if they have acted poorly.
Once you have their final response, you can make the complaint to the Ombudsman here :
www.financial-ombudsman.org.uk/consumers/how-to-complain
Or by email:***@******.***
Based on the circumstances, assuming the bank does not uphold your complaint, I am sure the Ombudsman will 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
The above is the only way forwards in my view, short of court action as I mentioned earlier.
A lawyer is not required - the procedure is set up for consumers and small/medium sized businesses as a means of alternative dispute resolution.
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).
This is considered as a last resort though - I am sure the Ombudsman will resolve this given the facts.