Thanks for the reply irishguy28. I had tried the method that you mention but it always results in an error message - even when I put in flights on which I know there is availability. I have found BA's website to be somewhat less than reliable in the past so I suspect that they don't have this functionality working correctly at present.
For those interested, I am trying to use the website to make the following cash booking, in club class and for 2 passengers:
Sat 21 Dec: London - Hong Kong
Mon 23 Dec: Hong Kong - Melbourne
Fri 10 Jan: Melbourne - Singapore
Sat 11 Jan: Singapore - London