As has been said a couple of times .. but not IMHO emphasised enough ... The DVLA 3500Kg limit will rarely apply to MOST folks who do not use a 4X4
The "normal" limit will be the vehicle train weight off the VIN plate. If this is less than 3500 kg then the DVLA limit matters not. If you exceed the Listed train weight you are breaking the law.
If the listed train weight is greater than 3500 kgs and you don't have B+E then 3500 kgs becomes your limit. If you do have B+E then the listed train weight will always be the limit.
The listed train weight is made up of the maximum weight allowable for the vehicle, also on the VIN plate, and the maximum allowed weight of the trailer as given by the trailer plate.
The "law" that stop you.. be it police or VOSA, do NOT have to weigh you on-the-spot, or take you to a weigh bridge. They only have to read the two plates,do some very simple sums, and maybe check your licence.
I have deliberately tried to avoid all the acronyms of MAM, MPTLM, "Kerb Weight" etc... it is the plated weights that matter as that is what the "law" will read.
http://www.btinternet.com/~madmole/images/VinPla2.gif
Item 3 is the maximum vehicle weight
Item 4 is the maximum train weight
If 4 is less than 3500 then the DVLA limit is superfluous for both B and B+E drivers. Item 4 is the limit.
Subtract 3 from 4
The answer is the maximum allowable trailer weight. It must be greater than the maximum weight figure from the trailer plate.
Finally, the maximum trailer weight from the trailer plate must be less than item 3.
The answer is the maximum trailer weight