Yeah I think you guys are right about 150 kg being a bit light on. Would have to be more than that!!
Re extra Jerry's, taking into account the other replies, it would seem the best solution would be to add an additional one up front, giving 2 Jerry' total. Tow Ball weight would be getting close to 200kg though, with 2 Jerry's, full gas and front boot full of gear to add to the stamped weight on the compliance plate.