Do I need to pay national insurance?

I write books for an overseas publishing company as a freelancer (NOT an employee) and receive royalty income every 6 months. Am I regarded as self-employed? Do I need to pay NI? Kindly advise.

Hi

Yes. As a freelance writer, you are self employed as a writer.  
You will need to register as self employed and declare the royalties as turnover of a self employed writer.  
The profit after deduction of expenses, is taxable and subject to Class 2 and Class 4 national insurance, if the profits exceed the thresholds.  
Please have a look at Check how to register for Self Assessment
and Rates and allowances: National Insurance contributions.

Thank you

In fact, I receive both royalty income and interest income from savings. As said, I've already registered for self assessment and got a UTR. Do I need to register for anything else, like a business tax account? Can I use the same UTR to report both types of income in my tax return?

Hi
You can report both types of income on the one tax return.