Hi Callum,
Where an individual who is self employed and employed in the same tax year, Class 1NIC on employment income is calculated first, followed by Class 4, taking into account lass 1 NIC already paid.
Where you have paid too much NIC, you will need to seek repayment via the NIC helpline at 0300 200 3500 (Monday to Friday: 8am to 6pm Closed on Saturdays, Sundays and Bank Holidays).
More information can be found at:
National Insurance: detailed information
Thank you.