.... the issue is that the MMB seat map doesn't map on to the actual seat map. I tried changing to 'F' but it just comes up with an error - but clearly others have been able to grab seats. Also phoned BA Gold line, but they just got an error too: there must be a way of doing it!
UPDATE: as always, never accept the first 'bid'. Called BA again and that agent managed to change seats to 3K/4K - no problem. I think the first time I phoned I ended up with the Indian call centre, the second time it was the UK one... .