I would beg to differ a M3W is NOT derived from a four wheel body, it has a motorcycle engine and therefore more M/C.
The DVLA document is obviously open to interpretation, and a better description would be do you sit on the tricycle or in the tricycle. If on then it is motorcycle derived if in then it is car derived, MMC clearly think it is car derived as all their UK dealers fit front number plates on delivery. If as you say it is motorcycle derived then all those cars are illegal according to DVLA as they should not have front plates.
There is sufficient confusion over this that I am sure no one will be pulled for either having or not having a plate. Come MOT time however it is required to go through a Class 4 test which is for a car, they only exception in this test is it is exempt from emission testing.
