Thank you all for your input - I can only access the booking through the BA MMB site, and for ages the seat plan for the flight said A34D, but when I went back the other day, although the system still said it was the airbus, suddenly there was an upper deck available to choose! Also the departure time listed on the BA site is slightly different from the actual one, so I guess their systems haven't updated properly to reflect the new equipment and flight schedules.

At any rate, the whole journey (LHR-HKG-AKL-SYD-BKK-LHR) was booked in F and I was told this was not available on the CX sector, so hopefully will get TA to rebook me into F, which should make the journey more comfortable.