If its for your last tax year you can claim 40p per mile for the first 10,000 miles and after that its 25p per mile.
The allowances have been increased for the current tax year.
There are two ways of claiming and you have to decide at a certain time which method you are going to use. I think you can only change when you change vehicle.
one method is to claim the 40p a mile, this covers all expenses relating to your vehicle, road tax, petrol, repairs and maintenance etc.
The other method is to claim every item seperately.
So if you do high mileage its probably best to claim the 40p
for both methods you must keep records to prove.