If I am registered as self employed and then start working say 16 hours a week as an employee for another company, how will it affect the tax that I pay?
You will need to keep all your payslips and declare it when you fill in your self assessment, I don't think it will have much bearing on the tax situation as you will have paid tax on your 16 hours a week and self employed tax will be calculated separately.
I asked this question some time ago and was told that if the self employed earnings made a loss (after setting against it some household an travelling bills), you get some PAYE money back.