Hi,
If you are working as Self Employed in your second job and are registered for Tax and National Insurance as self-employed, you should be completing Self -Assessment returns. If you earn above the Small Profit Threshold for paying NI, you will pay Class2 NIC through Self-Assessment.
If you do not earn enough (you are below the Small Profit Threshold) you may be able to make Voluntary Class2 NICs to maintain your NI record for Pension purposes.
You can find out more about being Self Employed and Registering for Self-Assessment at:
Register for Self Assessment
Thank you.