Most european countrys road tax is integrated on the actual licence plate and is not a seperate documant that you have to fix to the car or bike, on german number plates for example it is a small disc that is adherd to the actual number plate the road tax and registration are dealt with through the police, failing to pay your road tax invites an visit from the local police who promptly remove your licence plates.
In Denmark, it is the same only no disc attached to the plate, you can only get number plates from the police station and new plates are requierd everytime you purchase a car\bike as they are registerd to the owner of the car\bike and not like in the UK an indication of the year of registration.
This system seems to work well as you cant get a number plate without you have paid road tax, taking the car\bike off the road is simply a case of removing the plate and dropping it off at your local police station, when you want to ride drive on the public road again you simply pop down and pick up a new set of plates,
This also goes for insurance, the insurance company inform the police if a registration is not insured, if the plates have not been handed in to the police you get a visit