Yesss... but it was you that brought up the suggestion of going via HKG in the 1st place

For a straight LHR-SYD flight , I would say that whatever you do , likely to be screwed up by being up for so long and arrving in the morning anyway, that just to sleep when actually tired rather than having an unpleasant time just trying to stay awake

Assuming staying at a hotel in Sydney, then would suggest ensuring that have ensured an early check in or booked from night before so as to be able to get some rest on arrival after the anti social arrival time ot 5am