Lee I can see where you're coming from and to a point I agree with you, based on the fact that you pay for the vans and fuel and so on. All i'm saying is just be careful to think about it from your staffs point of view, many people I know (including myself!) have been on the receiving end of the "GPS question" and from experience, it does nothing to help motivation/moral. Now I know you're not fitting it to increase motivation/moral, however, these are important things to consider when weighing up the pro's and con's.
That said, if you get it fitted, good luck and I hope it works for you.