With regard to TOP's question, it's a bit like your consultancy fees Anna. Though I suspect most customers would not be happy to be charged exhoribtant travelling expenses if they were already being charged the going rate for the job (Read might go and ring up the paragliders in YP and see what they charge for Lime plastering :=)))
Here last time I checked they were paying 48p/Mile for a 1.4L engine and even more for gas guzzling sin-mobiles. For most trips (not that there's many :=(( I take the train.
I could have made a nice "profit" (based on the premise that most of my fixed costs are already paid whether I take the car or leave it sitting on the drive) on a round trip to Worster, but i'd have lost my street cred as an eco-warrior. Actually, I think on that trip I could probably have hired a car for my wife to drive for the week, and still made a profit!
I think the policy of my last school was much more sensible. The Bursar there would pay the second class train fare. If you chose to take your car instead, it was your loss/gain.