It is unclear from the question if you have Nationwide's final response to your complaint or not. The dispute is covered by the Financial Ombudsman Service, so the only way to resolve this is to a) make the formal complaint to Nationwide and b) assuming they do not uphold your complaint, ask the Ombudsman for assistance.
You are covered under the Consumer Rights Act 2015 here which means you have a right to expect a service (from Nationwide) carried out with reasonable care and skill. Furthermore, the account 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.
If you have not done this already, 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.nationwide.co.uk/support/contact-us/make-a-complaint
Once you have the deadlock letter the next step would be to escalate this to the Ombudsman - they will investigate and liaise with Nationwide which would hopefully result in a resolution. They can order Nationwide 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 Nationwide 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 Nationwide and I would fully expect them to resolve this for you.
Please note we cannot do this for you as we are an internet forum and not a law firm - though the complaints process above is designed for people to pursue it without having to instruct a lawyer.