HMRC not accepting employers EYU

Hi In tax year 20/21 my then employer significantly overpaid me, which I have repaid. This significantly impacted my tax codes for the following year and I believe I will be owed a rebate from HMRC, as HMRC still believes my salary for 20/21 was several thousand pounds more than it really was. My tax code for 21/22 and beyond was adjusted to repay this tax on salary I did not in fact earn and do not have. My employer have been assuring me they have submitted an EYU - several times since March. However, this has not shown up on any HMRC systems and is not linked to my records. This employer has offered me the reference number generated on EYU submission for HMRC to track, however everyone I have spoken to at HMRC has said they are unable to use this number to track anything and still cannot find any EYU. This employer is no longer my employer - I left in 2021 (which was part of the overpayment issue) and they say that HMRC will not speak directly to them anymore as they are not my current employer. Could you advise me how to proceed to ensure I HMRC has the right adjusted income for 20/21 following my employer's error, and I have paid the correct amount of tax? Do I need to escalate this within HMRC?

Hi Eleanor,
Do you have any documents provided by you ex employer showing the overpayment and also the adjustments made since to pay this back? In first instance  I would try speaking to out helpines to see what we do hold again to see if anything has changed and if so we should be able to deal with it then. Failing all that you may need to send in the documents of what has happened by post so we can try and resolve that way instead. But obvioulsy this is a more lengthy route.
Thank you.