CSLM13015 | FAQs: forms questions and answers: where do I record SL deductions?
From HM Revenue & Customs · Collection of Student Loans Manual
Suggested answer
Payroll software will take care of student loan and Postgraduate loan deductions for you and they will be made and recorded automatically along with any regular tax and NIC amounts.
HMRC action
None.
Note: The employer should show the amount deducted each pay period on the employee’s wages slip.