Just to note that depending on who issues you your temporary number, you may have to notify your tax office that you worked under a different NIN so you can get your refund. I started work in the UK under an employment agency and they issued me a number based on a formula of TN (temporary number) +date of birth+ gender. I then had to write to my tax office to let them know I had changed numbers so I could get that tax back as well. If your temp NIN is issued by the authority, you shouldn't have the problem, but you need to check and be sure.
Also, you can write to the tax office at anytime to request a re-assessment of your tax bracket, if you feel your employer has not done so correctly. THis has saved me loads of money.
