The only way to get an exact figure, for your particular flight, is to go to the relevant airline website and seek flight details on the relevant date. Sydney is normally 10 hours ahead of GMT but Daylight Saving Time will be in operation during November, so there will be an 11 hour time difference.
As a rough guide, the 1115 Qantas flight from Heathrow, on 14 November, is scheduled to arrive in Sydney at 1930 on 15 November. Because of the 11 hour time difference, it will be 0830 in London when the plane lands. That makes the total flight time 21 hours 15 minutes.
Chris