National Insurance overpayment due to PILON

In September last year, I lost my job and was paid PILON. As it came in one lump, it looked like I was being paid more than I was. I was able to claim the tax back, but my former employer has said they cannot return the overpaid NI and I was refused when writing to HMRC. I didn't have another job until March 2024, so definitely overpaid NI, how can I rectify this? I really need every bit of money I can get.

Hi,
Please see further information regarding PILON’s on the gov.uk website which confirms that this type of payment is treated as earnings for Class one National Insurance purposes -
NIM02520 - Class 1 NICs: earnings of employees and office holders: payments made on termination of employment: payments in lieu of notice (PILONs) : General
After reading this information, if you disagree with the reply that you have already received from HMRC, you would need to contact them in writing to explain why you disagree.
Thank you.