Wildy:
Thanks. I’ve now obtained a copy of the dangerous goods note. Total weight of goods was 3700kg.
Hi Wildy,
So after all that… the UN number wasn’t required to be displayed on the container anyway!! 
4 X UN Class 4.1 placards were all that were needed on the container by Sea, Rail and Road.
Once the container was loaded onto your vehicle, you needed a pair of the standard ADR plain orange plates one on the front and one on the rear of your vehicle.
Wildy:
So the 8ton I saw must have included the weight of the container.
That would be my guess too!
Wildy:
However I now believe there to be other issues with the note.
Quite possibly if the placarding was anything to go by.
Wildy:
Also surprised that the box travelled from China without anybody noticing the problems.
I cant speak for Chinese standards, so all I can say is that China should adhere to IMDG.
My surprise comes at the lack of a quite fundamental and basic placarding issue being spotted at the UK port of entry AND the railhead people also missing it. 