Income and Corporation Taxes Act 1988

2 (1) If, for any part of the relevant year, the car was unavailable, the cash equivalent is to be reduced by an amount which bears to the full amount of the equivalent (ascertained under Part I of this Schedule) the same proportion as the number of days in the year on which the car was unavailable bears to 365.

(2) The car is to be treated as being unavailable on any day if—

(a) it was not made available to the employee until after that day, or it had ceased before that day to be available to him; or

(b) it was incapable of being used at all throughout a period of not less than 30 consecutive days of which that day was one.