I am seeing all these remarks zipping around and people keep reposting the definition of MZFW. The whole point of me adding to this thread and livening it up again was the attachment I found which explains centre tank fuel being used in reducing operational payload and what exactly is happening with wingroot/spar forces. Information that had not been alluded to before in the thread. Yes we all know ZFW doesn't include fuel, but that was probably invented before aircraft were designed that had centre tanks!
Chu Chu, yes correct, as per the attachment I put up in my post.