When are trade plates required?

[Paul C 75]

I know all about the restrictions of driving with trade plates. My understanding was that trade plates are used to "cover" the fact a car isn't taxed on the road. BUT are they also required when a car is "in the trade" ie it doesn't have a registered keeper? For example, if I wanted to drive an "in the trade" car to/from an MoT, do I need them, since I don't need tax because there is a specific exemption there.

TIA!

[Willoughby 15]

My understanding is that if you are moving the car on the the road with no tax - you need to display trade plates (whether its in the Trade or not)  as you say ... if car is pre-booked for an MOT they aren't needed as this is a specific exemption....