They have to make sure they get a VAT receipt for all their fuel purchased to be able to prove they've bought sufficient fuel to justify the mileage and then there is a formula which you can find on the HMRC website for the amount of fuel content in the 40p (which is now 45p in this new tax year). For a 2 litre diesel it was 11p of the mileage that related to fuel content - the balance being to contribute to depreciation, servicing, tyres etc
So, if your total mileage is 1000 miles at 45p = £450 expenses. Of this 1000 x 11p is for fuel = £110 - this then includes VAT at 20% so divide £110 by 6 = £18.33 vat claimable - but you have to have the receiptsl