You can calculate their business-related motoring costs by either:
a) Take the proportion of business miles to total mileage driven in their vehicle in the year and apply that proportion to their total motoring costs for the year; or
b) Use the fixed expense of 45p per business mile for the first 10,000 miles driven in the year and 25p per mile for additional business miles in the year.
If your client uses method a) they can also claim capital allowances on the cost of your vehicle, restricted for the private use of that vehicle. However, if they use method b) they can’t claim capital allowances for their vehicle but they can claim the interest amount of any finance lease used to purchase the vehicle.