Thanks for the suggestion but I haven’t paid any tax in the UK since 2021.I've been doing SA for 25+ years and invariably my income tax is overpaid and they owe me a refund. It used to be the case that you put your bank details on the SA for where any refund is to be sent. In the last 2 years they have reverted to sending cheques. One last thought, are you sure they've not "returned it" via a Tax Code adjustment? Unusual for a credit but if they have, you can demand the full lump sum.
I have called periodically and the somewhat standard reply I now get is we’re behind with repayments, I will look into why it has not been repaid, and call you back in a few days. Then silence. I have a long list of numbers, dates, length of call and person I spoke to.
I will give it one last try over Xmas holiday and then see what options are open to escalate it.