144Deduction for payments for private use
(1)A deduction is to be made from the provisional sum calculated under step 7 of section 121(1) if, as a condition of the car being available for the employee’s private use, the employee—
(a)is required in the tax year in question to pay (whether by way of deduction from earnings or otherwise) an amount of money for that use, and
(b)makes such payment.
(2)If the amount paid by the employee in respect of that year is equal to or exceeds the provisional sum, the provisional sum is reduced so that the cash equivalent of the benefit of the car for that year is nil.
(3)In any other case the amount paid by the employee in respect of the year is deducted from the provisional sum in order to give the cash equivalent of the benefit of the car for that year.
(4)In this section the reference to the car being available for the employee’s private use includes a reference to the car being available for the private use of a member of the employee’s family or household.
(5)This section is subject to section 145 (modification where car temporarily replaced).