Hi Mark Jones
If you are an employee, your employer will deduct class 1 NIC from your earnings.
If you then become self employed in addition to being employed, you would need to register for self assessment as self employed (
Set up as self-employed (a 'sole trader'): step by step).
If your net profits exceed the threshholds, then you will be subject to class 2 and class 4 NIC.
Where this is the case, your class 1 NIC deductions will be taken into account when calculating your class 4 NIC please see
National Insurance: introduction.
Thank you