Trojan1981
I am surprised that you were able to get a good price in August for BA - everything I've seen is at least £1400 pp on BA/QF - may I ask what you found?
As angels pointed out, the key thing to your comfort is breaking your trip. Two nights in BKK/SIN or KUL will chill you out (wrong expression there) and effectively give you an extra day in Australia.
I usually take the night flight from LHR to BKK, stay for three nights and then take the redeye to SYD, where I get a full day in. For some reason it's easier for me to come back non-stop, tidy up and go to the office.
I'm 190cm tall and it's damned uncomfortable, but I'm not dead yet. I declare a bias here because we have access to BA/QF lounges which makes a hell of a difference. If you have no allegiance, it sounds like MH if the stopover is not an issue.
Sea Oxen