im sure its to do with total mam's when it comes to towing on an later type licence (what i have)
total mam of car and of trailer must be less than 3500kg (mam= maximum alowable mass)
i asked a traffic copper about it and he said i could not tow a trailer! then i showed him the back of my licence

but no one has been able to give me a stright answer and some trailers dont have marking so what happens then?
good thing is i can tow a trailer of any size with a motor bike or trike after i passed my bike test
