unfortunately it is the case now. if your train weight exceeds 3.5 tonne then a tacho has too be fitted. that is the law regarding trailers. but the reason you see lots of people with transits towing this and that is simply that there not bothered.. but i've towed heavy stuff, its normally 'in a coppers eyes' If the vehicle and trailer looks right they will leave you alone. I.E. if you have a transit towing a trailer with a transit on it then it is obviously over the towing capacity of the tow vehicle. the trouble is that to have a transit, you then own a commercial vehicle, so its seen as hire or reward. and if you run your transit on a traders policy that also is commercial. if you see what i mean. like I said earlier, if you keep the towing weight to about 70% of the weight of the tow vehicle I don't think you will go far wrong.