aristoph - OK, I guess threre's 2 issues here maybe. First, not letting you move - did you get seat shifted at all so far perhaps? Second, the dodgy 757 which doesn't correspond to the normal seat map.

As advised earlier, I would call BA and have them delete your seat assignments totally, then re-add them manually where you want to sit (e.g. 3AC). That should solve the first problem and also the second in that you know not to try for 2AC.

There really does seem to be a bug if 'the system' gives you a seat itself in some situations - most usually post-shifter action (the curtain or plane change type anyway). Even if you don't think you've been shifted, perhaps behind the scenes you were meant to have a 'normal' 757 then it got swapped to this one and your seats were transferred but the system still doesn't like it. Bizarre, but I can see it happening...