Thanks again both of you.
The flight is planned as the first stage of getting back to the UK, but as we are hoping to be using an Amex 241 voucher, we need to return on BA. This option is a backstop should we be unable to get SYD-LHR-MAN flights in CW/First for our return leg, as we could still get CW/First from NRT whilst (hopefully!!) snagging first outbound for the full route.
We would also need to depart from NRT as both Qantas and JAL only serve NRT from SYD/MEL, so the plan would be get the daytime flight and overnight in a hotel to try and get a decent night’s sleep. Another advantage of the Tokyo flight would be that it is a daytime flight, so hopefully jet lag should be reduced.
Once again, many thanks for your advice - greatly appreciated.
Our other options are, as mentioned, use Cathay Pacific to HKG then BA in CW/First, or pay for flights on Scoot from SYD-SIN then pick up a BA flight back to the UK.
Anyhow, it looks as if the problems I was having with the BA website (I assume it may have been my IT) have gone away now, and I am able to see JAL flights.