A few hours later and I am now able to select a seat for myself in MMB. What is interesting (for those of us who mainly travel in Y on long haul) is that on MMB the last row of the plane appears to be row 53, but on both BA's own seat plan page on BA.com and on seat-guru, rows 54&55 (seats DEFG) are shown for both variants of the 744. Any idea why rows 54&55 aren't shown on the seat selection page? I'm sure I've sat in 54D before.