montana man:
Maximum Gross Weight (MGW)
The maximum figure set by the manufacturer for the gross weight. This will normally be the technically permissible maximum based on the carrying capacity of the tyres, axles, coupling, suspension and chassis but may have been adjusted downwards for commercial reasonshighwaycode.gov.uk/sign059.htm
Thats an interesting quote 
So my Belgian registered truck running on 5 axles is perfectly legal to run at 42 tonne and even to carry 13.5 tonne on the drive axle because that is what the manufacturer has decided.
I would go by Denis F interpretation and argue the toss later.
maximum gross weight is the same as the plated weight - if it says 18t mgw you
don’t take an artic over
If this is a local council putting these signs up then I suggest they check the difference between plated weights and gross weights.
18T gross can only mean a 4 wheeler