When you first start using your van, you have 2 chooses with regards to the taxman
1 Mileage rate
Its 45p per mile for the first 10000 miles then it drops down to 25p a mile.
You cannot claim any expenses incurred for running or buying of the van.
2 Expenses
You can claim any expense the van incurs, from the van cost, fuel, mot and road tax, down to a lightbulb.
Now which ever method you use, you cannot change over to the other method, whether its in this tax year or the next, you can only change if you replace the vechicle.