Avoid trying to sleep on the SIN-SYD sector. The meal service will take 2 hrs to run, the breakfast (if an overnight flight) will start 2 hrs before landing and that leaves a bit over 4 hrs for sleeping. Not good if you really need sleep. I'd much prefer to do LHR-BKK-SYD or LHR-HKG-SYD as the run down to SYD will allow 2 more hours of sleep if that is what you are aiming for.