I'm self employed and the Inland revenue said you can either keep your fuel receipt and any other receipts for repairs to your vehicle etc or keep record of the mileage.
It works out better for me to record the mileage.
40p per mile upto 10,000 then 20p per mile there after each year.
Claiming the mileage means you can't make any other claims on your vehicle as the 40p per mile is supposed to cover this.
That came from my local tax office and the Inland Revenue when i spoke to them on the phone.