I asked the question on another forum http://caravanersforum.com/viewtopic...50896#p1150896

To be honest I'm still not clear either but it seems (to me) that perhaps you were correct in the first place. ie. That the GVM is derated by 220kg & then the ball weight has to be included as well. Bummer!.

May need a GVM upgrade (which as I understand it will reduce the max weight you can tow by the increase in the GVM weight as GCM mus remain unchanged) .... so in this example the max weight of van you could tow would be reduced to an ATM of 2765kg IF you loaded the Patrol, with upgraded GVM to it's full extent. Still not a lot to play with but it does offer a bit more choice as to where the allowable weight gets stowed.