I'm finding it amazingly difficult to get any UK authority to face up and tell me how they think the UK derives its authority in Shetland. 

Having put a vehicle on the road with no tax or MOT and having admitted driving it about 15 miles on one occasion and about 3 on another, the police have decide not to prosecute me!

In view of the fact that simply not displaying a tax disc is sufficient to attract a £40 instant penalty, they have decided that is a matter for the DVLA (the vehicle licensing authority) and because I was not driving the vehicle at the time, they could not prosecute because there was no corroboration. How far will they bend the rules to avoid a confrontation? We will have to wait and see.