If your company provides you with a company car but you do not get private fuel costs provided as well, you can claim a fixed amount per mile depending on the cc capacity of the car and whether it is petrol or diesel. The fixed amount you should claim for business mileage has changed and the new rates can be found here
P.S. The tax you have to pay personally and that the company has to pay if private fuel is paid for by your company is absolutely penal. It is calculated that you need to be driving approximately 20,000 private miles a year to make the tax benefit worthwhile. To avoid the fuel benefit, you must ensure that the company does not pay for a single drop of private fuel during the course of the tax year. To ensure this, it may be necessary to pay back to the company any fuel accidentally claimed through the company using these same new rates.
The rates are updated by HMRC every 3-6 months as fuel/diesel prices change, so keep looking on our website for details of future changes.