Payroll Amendment older than 7 years

Hi HMRC Team, We have a client who has identified that some employees were not moved from NI Cat X to NI Cat M and there has been an underpayment of employers NIC. The issue goes back to 2016/17. HMRC Basic Tools only goes back to 2017/18. How can we submit the amendments to you?

Hi
Please write in to 
PT Operations North East England
HM Revenue and Customs
BX9 1BX
United Kingdom

providing your correct year-to-date totals to allow us to investigate.