Private car used for business use, Tax questions?

I use my private car for business use and accept Payment in Lieu of a company car. I also have a fuel card. I heard a couple of people at work who do the same say that they get a rebate each year because they use their car for business use. What are they talking about? I understand that I may be able to claim some money back from the taxman but how and when i don't know. Any help would be great!
I don't pay any contributions to private mileage as i think i am taxed already. I had to declare private mileage in april and was told that it was for tax reasons. i haven't been keeping a real record either.

If you are being reimbursed for the mileage, then you can not claim anything on your tax return.

You can only claim UNREIMBURSED business expenses.

Your question is sort of confusing. First you are paid in lieu of having a company car so that money is taxed. The fuel card you didn't say if it was your card or the company. If you are using a car for unreimbursed company milage you can deduct this on your tax return in the area of business deductions.
it for the miles you use on your car ask the person that dose your taxes. Do keep a record date and were you went and distance.