Thanks for your help. For some reason checkmytrip has not got any of the seats attached to it. However, I went back into the PNR on BA and re-confirmed the seat which seems to have done the job. For my outbound I want to select 61J, which should be available - at least for purchase. I called BA yesterday and they said showed up as blocked and not selectable for me on their system too. their advice was to wait until T-7 (which I'm assuming is linked to the departure time?) and give them another call if it still wasn't showing.