NJC,
If your going to use your mog when you get to your worksite that's ok in my interpretation of the regs or if your travelling back home, the fact that your towing a trailer as well with a piece of equipment on is immaterial as you cannot move a load of soil / woodchip / use the winch around a worksite without your mog, that's why I justify taking it there. As for privately owned woods, well their owned by farmers around here so surely your farming when you go in them!
I know if I was to do an emergency stop I'd rather have a unimog right behind me at 50mph than a transit tipper fully loaded with a tacho and trailer, as I'm sure I'd soon be wearing the tranny in the rear door where as the unimog is built for dealing with speed and towing, and I'm not aware of anyone getting pulled and checked around here for diesel or class of vehicle, and as farmers are the most compliant people in industry, their bought up with taxing vehicles, following the rules, it's not really worth taking the time to check out what fuel their using. I have a friend who worked on a farm and they got through 7000 litres a day of red in harvest, and they never got checked out.