Thanks for all the suggestions, especially like the idea on a cruise ship, but as I do have work to do, the boss might not agree on that one!
After speaking to BA, they have offered BA to SIN, with a stopover, and then Quantas onto MEL, then a few flights around Australia, with a return again on Quantas to HKG, and then BA back to London for £3,800 which isn't bad, considering I paid over £500 to do a return to Frankfurt on Wednesday.
Apparently the big saving is to book all the flights at one, as that way the internal flights are at a big saving.
I did look at the round the world option, but it does come out more expensive in the end.
AAF