An employee obtains a Tax Credit Certificate from their Tax Office and the PAYE calculations are based on this certificate. This certificate contains details of your tax allowances and reliefs.
The tax credit certificate is a very important document, as the employer will calculate the monthly PAYE due based on your tax credit certificate. If this certificate is incorrect you may pay too much PAYE. The Tax Office are the only ones who can amend your tax credit certificate and it can only be done during the tax year.
If you think there may be an error on your tax credit certificate you should contact your local tax office immediately. You could be due a tax refund if you are not getting the correct tax credits. Alternatively if the tax office have over allowed your tax credits you could end up owing tax at the end of the tax year.