Apparently, if a company finances a car be it by car allowance, lease or straight company car HMRC requirements are that the car should be no more than 4 years old.
Many companies insist on the 4 year age limit and require the car is replaced whereas others simply give you the car allowance with the caveat that 'It is up to you to ensure you have a vehicle at your disposal' meaning if you have an older car (or even one less than 4 years old) and it breaks down then it's down to you to find transport to carry on working.