Wrong payment history affecting previous tax year

I left a company on September last year and got my last payment on September. I got a payslip on October but didn’t receive the money. The payroll company noticed the error and reran the payroll so my p45 is correct, but the record in HMRC PAYE tax system is incorrect. In HMRC system it says I had got the October and now asking me for the tax which I shouldn’t be paying. I told the payroll company, they only provide me with the FTS transaction if and p45 form. But when I phone HMRC at 0300 200 3200 they told me as an employee I could ask HMRC to correct the error. What should I do now? Can anyone guide me on this ? It’s causing a loss of 800 pounds to me!!!

Hi,
HMRC records ar only updated when the company submits revised figures for the year, if they are unsure how to do this
they need to contact our Emplyers Helpline.
Thank you.