Le Chat is missing the point as I am sure you appreciate. I would guess you no longer want the company to pay your private fuel because the tax is more than the value of the benefit, and it is a fixed sum for a specific car.
There are a number of ways to achieve this. Some suggestions - pay for all of your own fuel and claim back the business mileage at a pre-determined rate (the IR have some guidelines for this), or the company can pay for all of your fuel and you then pay the company for your private fuel at this rate. I do the former, and recently my company has now agreed to pay me a sum (�50/month) in lieu of private mileage so I win both ways!
Note - I have done the sums for buying my own car, and it is not clear cut. There are calculators on some web sites. Google "company car tax" and one should come up.
Most people now accept that fuel benefit is not a viable option unless you do about 12,000 private miles a year on 40%.