OK, just got off the phone from the call centre.

Problem now resolved and we're back on the upper deck apparently. They were (as Cyba said) adamant that the matter would have been resolved at the airport etc, but it does make me wonder what on earth is going on.

The very helpful call centre rep was equally confused. There was a note in the system saying that it wasn't a suitable seat, but still showed with the bsct.

Oh well. Suggests that there has been no shift in policy, but at a whim someone decided that I shouldn't be sitting there...