This situation is specifically covered in the tax legislation. If a temporary car is provided while the principal car is unavailable for a period of less than 30 days, there are technically two cars available to the employee. However, as long as the replacement car is not "materially better" than the normal car, i.e. not materially better in quality or in taxable value, the employee is not treated as having two cars. There is no requirement in this situation for the temporary car to be reported and the normal car should be treated as being still available to the employee up to the date that the new car is made available.
HMRC guidance is on page 31 of the 480 booklet.
Ian Congreave retired from his payroll and HR work at the end of November 2011. He founded the Forum in 2004, when it was known as the PayPerShop Forum. Ian would like to thank the many Forum members who have asked and answered hundreds of questions and helped to make the Forum a valuable resource to the payroll community.