Some time ago, I worked for an independent charity providing healthcare.  The rules on car driving for work purposes were as follows: car drivers (nurses, doctors, social workers and senior admin) were specifically authorized to drive their cars for work purposes.  The cars had to be in a roadworthy condition.  Mileage was paid (according to size of engine, monthly on submission of signed mileage cards, countered signed by the person's manager) according to a certain rate AND a lump sum, paid annually (according to size of car engine) which assisted with maintenance.  It worked fairly well, and no-one actually "made" money out of the system.  

Raindance