errr.....
<<i had always thought that it was slightly behind the main gear,(only guessing) >>
I think you'll find that if the CG is behind the Main Gear, it will sit on its a*se! By definition, in a tricycle u/c aircraft, the CG will be forward of the main gear
<<Out of interest, what is the most aft CG position on a such a big bird in comparison to an external feature of the aircraft?>>
Well, the main gear as an absolute limit!
NoD