Using a tachograph or not is all about the 'gross train weight' of the combination i.e. the towing vehicle and trailer, when being used commercially, which I do for my business. Anything over 3500kg and it's a legal requirement.
My Discovery kerbweight is 2010kg (base model - so no air con, 7 seats, air suspension, etc. measured on a weigh bridge) so with my 3500kg gross transporter attached means I can be running at just over 5 tonnes, hence the need for a tacho to ensure everything remains legal and fully insured, just in case
Strictly speaking, I believe all 3.5t Transit type transporters that are fitted with a towbar, even if not actually towing a trailer, should have a tachograph if used for business purposes. Most of these have a max payload of 1300-1400kg, which is nothing more than an new Astra or Focus. A 75/ZT is cutting it fine, especially if it's a diesel or V6. Towing weight is generally around 2000kg on these, so once again, they should only ever tow small to medium sizd cars if using a twin axle trailer at 600-800kg.
It's rare that I ever get up to a total weight of 3.5t for my trailer, but will often be at 2500-3000kg.
Believe it or not, the Discovery TD5 can actually tow 4000kg if 'coupled brakes' are fitted. This was a special mod offered by Land Rover on Defenders and Discos, which added an additional pump to the engine and pipework to the rear, specifically to operate pneumatic brakes on the trailer, just like an HGV.
I've only ever seen pictures of the system and I think it was intended for large display type trailers with steering front axles, like you see at shows. The thought of 4 tonnes swinging off the back of mine fills me with dread...!!
Operator licences are also required for most business vehicles over 3500kg, but certain additional rules apply to 4x4's so this is not required in my case, kerbweight being under 2040kg - I had to explain this to a VOSA inspector a few years back, but after going away & checking, he eventually conceeded that I was correct (see the Road Traffic Act description of a dual purpose vehicle
). It's a complete minefield once you start getting indepth, and even those that are supposed to know the law inside-out can get it wrong....
