On the desktop site click to add the flight to your account again and when you get the "this flight is already linked to this account" message there should be a hyperlink "manage this booking" option. That gets you where you need to be to select seats etc.

Its a disgrace that we need to find these work-arounds to access fairly basic information on the site. It's not as though this is just a short term system crash or anything as MMB hasn't consistently shown my future bookings for well over a year now. How long does it take BA to get this basic functionality fixed