Pete, your boss has probably said that as if the company don't keep a record of who was driving, and find themselves in the situation you're now in (don't know who was driving), the company (or Director of said company) has to bend over and take the pain.

That's as I remember it as per company cars. I'm pretty sure it was for this reason the company I used to work for wouldn't let pool car drivers share driving for long journeys (i.e. whoever booked the car had to drive).

Not sure how that relates to plant, but can't see it being much different.

Just found this which backs that up, and may be of some use:

http://www.pepipoo.com/Section_172.htm