Cheers Paul,
Unfortunately I've already had experience of rolling a loaded 38T log truck, (when the front left axle mount on the tri-axle trailer snapped, whilst negotiating a downhill bend in a Scottish forest) putting the trailer and my Scania 113M on its side against a cliff wall

Since that day, every wobbly trailer gives me spine shivers.
Without meaning to offend, the load is also worth more a bit more than a load of hay, to me anyway, so the boss would never let me put any risk on it.
Struggling to get insurance for it, as its not an aircraft, as it has no wings or engines and its not a vehicle, as it has no wheel!!
Trying to get a company to insure it simply as an historic display item is near impossible.
Anyway, I'm currently researching the idea of a mid 90's TDI or 130 Landrover, as they have a 4oookg towing capacity. I just need to find someone who has one and can assure me that they can actually tow 4000kg at that age