A pay element is needed to suffer Class 1 NIC only that does not increase the gross for student loan.  This is per the HMRC guidance when raising P11Ds and reporting non-cash vouchers:

- add the cost of the vouchers to the employee’s earnings when deducting Class 1 National Insurance (but not PAYE tax) through payroll

It appears Sage cannot currently do this without the gross for student loan being increased too?