I had the same problem this evening. I am flying LAX/LHR in CW with my GF on Sunday. Upper deck is full but I have read that the EF seats on the lower deck are good for couples and that 20EF is the best of the lot. A couple of hours before T-72, I noticed that 20E (which previously had been shown on the seat map as being blocked) was now available. However, when I tried to select the seat, it reverted to being shown as blocked.

I called CS. Not surprisingly, the first person I spoke with wrongly claimed that bassinet positions are strictly for infants and could not be assigned to anyone else at any time (even at the airport). I asked to speak with her supervisor who thought the same thing but at least agreed to try to get me the seat after she saw on her computer that it was indeed available. After about half an hour on hold, I had 20E and 20F. I realize that we might have to move if someone flying in CW shows up with an infant but I think that is extremely unlikely at this point.