You should check that the tax rates within the Tax Regime that your Own Organisation is operating under are correct.

To do this, click Start > Configuration > Accounting > Sales Tax Regimes and from the list in the right-hand grid select the appropriate Tax Regime.  Remember, you can apply a filter to find the Tax Regime you're looking for quickly.  NOTE:  The code for the UK Tax Regime is GB and the description is United Kingdom.

The dialogue box that appears lists all the Sales Tax Codes used within that Tax Regime.  You can add new Tax Codes and control the percentage rate used for each Tax Code.

To add a new Tax Code click on the appropriate row and then on New Sales Tax Code.  Enter a Code (which must be unique) and a Description, then click on New Sales Tax Rate and enter the appropriate percentage rate.  You must enter a date in the Valid From column and, if you want, you can also specify a date when the rate is no longer applicable under the Valid Until column.  Remember to click Save & Close.

Similarly, if you want to amend the details of an existing Tax Code, click on the appropriate row, make the required changes and click Save & Close.