It's a commonly accepted safety precaution to keep the trailer below 85% of the weight of the car, and never exceed 100%. But that's purely advisory.
From a legal point of view, you can tow whatever the manufacturer says you can. This figure in turn tends to be dictated by the maximum weight that the car can successfully perform a hill start up a certain gradient (I forget what the gradient is.) When it comes to proper (as opposted to 'toy') 4x4 vehicles, this probably also includes the use of the low ratio transfer box.
Realistically, you could probably tow 3.5 tonnes with a V8 Land Rover, but I reckon it'd be dangerous to try and shift that load at the legal limit of 60mph. At lower speeds, the heavier load will be every bit as safe as a lighter load at higher speeds.
To Andy: I'm glad you got to the bottom of this, and thanks for confirming that
do actually send you a letter. So far, I'd only heard it second-hand, so I wasn't entirely sure that it was true.