NI Credits error

My NI contributions are shown incorrectly with HMRC. I work for NHS. after several phone calls I have deduced that my contributions have been logged under a former employee number. But I still can’t seem to get the details updated and I’m going in circles. There are 4 tax years in error How do I sort this once and for all and make sure tax returns going forward will be correct?

Hi,
You will need to ensure that your employer is using your correct national insurance number that should be on your payslips and the P60 for the tax year going forward.Your employer can amend certain tax years for you.
If the employer has submitted incorrect details to HMRC, you need to write into HMRC with copies of the P60’s for the tax years involved and we will investigate. Please speak to your employer first as they can amend the details through the payroll for certain tax years.
Please include your name address and national insurance number in a covering letter along with any P60’s for each tax year or payslips to:
Individuals Caseworker
PT Operations North East England
HM Revenue and Customs
BX9 1AN
Thank you.