ok let me have a go at this.
1. You need proof stating that you are only sending your car to install the headunit at the caryard?
2. You will need to prove that the ODO was reading 7.1km when you handed the car over to the car yard. ANd receipt for the fuel that you put in.
even if you make it to court the caryard people will say that the headunit was installed at the boss's home. In my point of view dont bother cos you got a weak case. Cos you dont have hard evidence to back you claims up for the ODO read out. But i might be wrong.
Look just treat it as a lesson so be more specific and detail next time.