Missing P45, incorrect / missing pay on account

Hello, At the start of the month my tax code was changed, it was changed to pay back tax I supposedly hadn't paid for the previous tax years. After checking my account, it appeared I had not finished my previous role and was continued being paid by them many months after I had left and no longer received payments. I left in July 2022 and on my account, it shows April 2023. At this point I have spoken to several people on the phone to HMRC and each time I am advised differently, to no actual resolution other than amending my tax code to BR. My previous employers claim their records are correct and they sent the P45. HMRC claims they never received this information. My previous employers won't get in touch with HMRC. HMRC told me to send in my P45 so they can resolve this but won't look at it until April. I fear this won't be enough as the previous year 2021-2022 also appears to have errors also. What am I supposed to do? I am being sent back and forth and nobody will help me, I am stuck in limbo with HMRC thinking I am earning double what I am, I have already lost over £1000 on payslips from this. If I were to send in to HMRC all my payslips and P45 from the previous employer with a covering letter explaining what is wrong, can HMRC look at this and update their system? Thanks

Hi Thomas Penny,
If your previous employer has submitted payment details in error to HMRC that you did not receive then they would need to submit the amended details to HMRC to update your record.
If you have contacted the employer and they are not sending the corrected details then you should send your payslips/P45 with a covering letter to review. 
Please keep copies of the documents that you send to HMRC. 
Thank you.