Repayment of Positive Balance on Self-Assessment

My wife has a positive balance of around £1,200 on her self-assessment account. HMRC has held this balance for over 12 months. We have tried multiple times to ask for a repayment via the online tool and the app both of which have been giving error messages for months. We have also tried to ask for the refund vie the phone line. A few weeks ago we waited for an hour for a call to be picked up and then the advisor hung up on us. I have now been waiting for an hour on a call which is yet to be picked up. Is there any other way to ask for this repayment, e.g. a form, by letter, etc?

Hi
This link explains how to contact HMRC including providing an address to write to Post