Okies .. a couple of little examples, using the VIN plate from a 2001 Elite Saloon .. YOUR CAR MAY BE DIFFERENT SO CHECK FIRST ..
These are my personal views as I see the limits...it is YOUR responsibility to check that you are legal.


Top line is the Maximum Authorised Mass for the Car .. in this case 2200 Kg
Second line is the Maximum Authorised Train Weight .... 4075 Kgs the maximum it + trailer can weigh.
Next two lines are maximum axle weights and although important play no part in this bit.
From the manual the unladen weight of my car is 1744 Kg
So ..
Category B licence holder :
May tow up to a TOTAL TRAIN weight of 3500 kgs, and the car has a MAM of 2200 kgs
3500 Kgs - 2200 Kgs = 1300 kgs
So it is legal for a Cat B holder to tow a trailer with a MAM of up to 1300 kgs .. note that is MAM .. not the weight ON the trailer .. it is the plated limit of the trailer.
This is because .. 3500 kgs has not been exceeded, the trailer 1300kgs is less than the unladen weight of the car.
Category B + EThe maximum authorised train weight is 4075, the maximum authorised car weight is 2200 so
4075 - 2200 = 1875
1875 kgs is the MAM of a trailer that I as a B+E holder can pull, again .. not on the trailer, but plated for the trailer.
Now, my caravan plate has a MTPLW (Maximum Total Permitted Laden Weight) which is the same as MAM of 1650 Kgs so I can legally tow quite happily..... on my B+E licence.
There endeth the lesson ...

HTH
