I would have thought it would be up to yourself from the begining wether you paid the drivers from when they pick up the keys from your office until they bring them back, or you pay them for the set hours you mentioned above.
Which ever way you go, you will need to have it in writting etc etc
I pay my cleaners from when they start loading up their vans to go to site till they arrive back and am finding that if the roads are very busy, then i am paying them for sitting in the van

Sometimes costs a lot of money, so there is that to think about before you make your mind up.
Kind Regards
Tim