Taken from the UK Revenue and Customs website:
Example 2
Flight details
London to Dubai then Dubai to Sydney
scheduled to arrive at Dubai UAE (Band B) at 17.50 on 1 November
booked to depart to Sydney (Band D) at 23.15 hours on 2 November
as more than 24 hours has elapsed between the flights, they are not considered to be connected for APD purposes and £60 or £120 APD (Band B rate) (depending on class of travel) is due.
The class of travel occupied between Dubai and Sydney is irrelevant.
London to Dubai then Dubai to Sydney
scheduled to arrive at Dubai (Band B) at 17.50 on 1 November
booked to depart to Sydney (Band D) at 23.15 hours on 1 November
as the scheduled departure of the second flight is within 24 hours of the arrival of the first then the flights are considered to be connected for APD purposes and £85 or £170 APD (Band D rate) (depending on class of travel) is due.
If a higher class of travel has been occupied on either the London-Dubai and/or the Dubai-Sydney leg, £170 APD is due.
Rates quoted in this example are for 1 November 2010 to 31 March 2012 and are for illustrative purposes only.
Have a stop over one your way back and save money
Or fly from the UK to somewhere in Europe you'd like to visit, such as Paris and arrange your ticket so you only transit the UK on your return and pay the lowest APD charge for a flight to Europe.
Case B covers international connections (a flight connecting through the UK to another flight to an international destination).
International flight to international flight
Case B rules
£0 APD due.
Alternatively buy an open jaw ticket SYD - DXB - LHR then PAR - DXB - SYD. Get yourself over to Paris quite cheaply spend a day or two and fly home from there.