Thanks both!
For anyone interested I had a chance to put the truck on a weighbridge today and it came out at 13,300kg. This is missing about 350kg of fuel, 100kg of passengers, 200kg of personal effects, and about 50kg of equipment, putting the total at around 14,000kg. This is fine by me, the total GVW is 18,000kg and front and rear axles of 8tonne and 13tonne respectively so I am well within weight limits. The weighbridge operates for a local waste management site and they were too busy to weigh the front and rear separately, I suspect that the front is at about 6tonne but I don't know for sure.
I wanted to keep below 15tonne total, although I wanted to be closer to 12tonne.