He must keep a record of all journeys for Business purposes, not including driving to the office - this is not classed as Business, and then he can claim tax relief on those miles. The current amount is 40 pence per mile for the first 10,000 miles and 25 pence for each subsequent mile. If he has not already been recording his mileage then hopefully he will have a diary or reports that he can refer to so that he can recreate these records. He should be declaring the amount paid to him for petrol and the �2,100 as income so that the tax office will then charge him tax on the excess if he has received more than the mileage allowance or give him a refund if he has received less. I have been doing this for a number of years.