Whoops - after posting have just seen almost simultaneous reply above from 419 - great minds think alike... etc. etc.
Going slightly off on this MOR track (and way outside my own expertise - all I did was search the net for the document; I have no connection with aviation other than as a passenger) for what its worth as background I just looked up the CAA document about MOR reporting (CAP 382 The Mandatory Occurrence Reporting Scheme):
http://www.caa.co.uk/docs/33/cap382.pdf
and Appendix B (Occurrences Required to be Reported) has in its list
4.3 b) "Incorrect loading of passengers, baggage or cargo, likely to have a significant effect on aircraft mass and/or balance."
There's no point dwelling on whether or not they should have raised one - either they did or they didn't and the time has passed but I do see from this where some of you were coming from in previous replies.