I'm with you on this - I do know a bit about this I'll confess......basically you need advertisement consent to display an advert and it is actually a criminal offence to display one without consent. However there are classes of deemed consent for which you do not need to get permission from the Council. One of these is the display of an advertisement on a vehicle. The catch to this is that it is not allowed if the vehicle is solely being used for the purposes of displaying the advert - i.e. if it doesn't move.

Could be difficult to prove and I suspect the Council would be completely disinterested in pursuing it but worth knowing. It's supposed to allow adverts on tradesmens vans etc but prevent static adverts on permanently parked vehicles.