It is vital that, as an employer, you show full and correct National Insurance numbers on all documents on which you record contributions, particularly when the forms are submitted to HMRC as part of PAYE compliance.
If an employee does not know their National Insurance number, you can trace it through HMRC by filling in HMRC leaflet CA6855.
If you have doubts over the accuracy of the National Insurance numbers that you hold, HMRC can check and update them for you.
The full procedure is set out in HMRC leaflet CA89.