from the Gov site....in theory the one that should matter i guess
Licences issued from 19 January 2013
From 19 January 2013, drivers passing a category B (car and small vehicle) test can tow:
small trailers weighing no more than 750kg
a trailer over 750kg as long as the combined weight of the trailer and towing vehicle is no more than 3,500kg (3.5 tonnes) Maximum Authorised Mass (MAM)
If you want to tow a trailer weighing more than 750kg, when the combined weight of the towing vehicle and trailer is more than 3,500kg, you’ll have to pass a further test and get B+E entitlement on your licence.
You’ll then be able to tow trailers up to 3,500kg.
Licences held from 1 January 1997
If you passed your driving test after 1 January 1997 and have an ordinary category B (car) licence, you can:
drive a vehicle up to 3,500kg MAM towing a trailer of up to 750kg MAM
tow a trailer over 750kg MAM as long as the combined MAM of the trailer and towing vehicle is no more than 3,500kg
For anything heavier you need to take a category B+E driving test.
Licences held before 1 January 1997
If you passed your car test before 1 January 1997 you’re generally entitled to drive a vehicle and trailer combination up to 8,250kg MAM.
This is the weight of a vehicle or trailer including the maximum load that can be carried safely when it’s being used on the road.
You also have entitlement to drive a minibus with a trailer over 750kg MAM.
Maximum authorised mass
Maximum authorised mass (MAM) means the weight of a vehicle or trailer including the maximum load that can be carried safely when it’s being used on the road.
This is also known as gross vehicle weight (GVW) or permissible maximum weight.
It will be listed in the owner’s manual and is normally shown on a plate or sticker fitted to the vehicle.
The plate or sticker may also show a gross train weight (GTW), also sometimes called gross combination weight (GCW). This is the total weight of the tractor unit plus trailer plus load.
reading that, it suggests/says that MAM/GVW is based on the plated weight and therefor you don't need a load to be in bother.
important parts BOLD that lead me to this-
Maximum authorised mass (MAM) means the weight of a vehicle or trailer INCLUDING the maximum load that CAN be carried safely when it’s being used on the road.
i fully admit you could read this either way relatively easily, hence why you got let off and a friend of mine got a fine for same thing.
some more here, can't comment on how official this is as it isn't a .gov site- http://www.towingtrailers.co.uk/rules.php
The "true" weight of the trailer is irrespective, so even if the HB505 is empty, it may not be towed by the Land Rover Freelander 2 TD4 as this would be an offence. Furthermore, it may adversely affect the handle characteristics of the steering, braking and cornering of the towing vehicle and, if a collision occurred, the driver may be liable for prosecution under the law of Construction and Use, as well as possible not being correctly insured by ignoring the maximum towing weight of the vehicle.