cjp
There are essentially 3 X 4hrs+ sectors:
London -> Bahrain (some of it subsonic)
Bahrain -> Singapore (again right at the aircraft range so 4hrs)
Singapore -> Sydney
As some pax would be on and off during the different sectors you will need to allow 1.5 hrs, for the turnaround.
Threfore we have 3 X 5.5 = 16.5 approx, it could be less, it could be more.