Thanks for the info everyone....
My thinking behind it would be that sometimes aeroplanes that can easily be flown across without ferry tanks are flown across and so the extra weight wouldn't cause too much extra hassle.
Other advantages for the ferry pilot
I can fly and have all the right tickets, so could do a stint watching the AP or flying while they grab a kip, someone to chat to (I promise to turn my head when they pee into the bottle
)..
It is catch 22, you can't do it unless you've done it (unless you do it uninsured)!
Cheers...