The Heathrow to Singapore flight indeed arrives in Singapore in Singapore morning 7:40, having departed Heathrow the previous morning. At that time, a SIN-SYD flight has left some time ago (20:30 the evening before) and has not returned.
No plane is due to fly back to London till 23:20, Singapore time - right?
And the plane into Sydney returns 13:45, Singapore time. From which time there are two frames in Singapore. Until 20:30 Singapore time, when they depart - 20:30 for Sydney, 23:20 for London.