Clare,
I have personally drafted the letters to get people off two seperate offences within the company within the last few months . They can ONLY prosecute you for with-holding the name of the person who WAS driving (if you are refusing to do so). If you are "seen" to co-operate (such as providing a list of names that it "could" have been), then they cannot do ANYTHING, as you have met all the legal requirements. Obviously this is easier to get away with if the vehicle is company registered and is insured any driver...

It is NOT a legal requirement for you to keep a log of people who drive your car , for businesses OR private individuals. So as long as you appear to be attempting to find the driver at the time, or can even provide a name (such as "Joe Bloggs" who didn't give his address on the test drive ), then despite lame threats about prosecuting, there is nothing they can actually do (without risking opening up the loop-hole publically ).

Mind you, they will probably do you for being an accessory .