In short, if you buy 2 separate tickets , then you will be liable to pay APD; since these will be 2 unrelated tickets, APD applies
You need to have a look at
http://customs.hmrc.gov.uk/channelsP...00505#P89_5226
In section 4.4 it shows the required criteria for the flights to be classified as connecting for APD purposes
Originally Posted by HMRC
4.4 Tickets
In addition to the time related criteria, the agreement for carriage must be evidenced by a ticket which must show the:
■airport from which the passenger intends to depart
■date and time of his intended departure, and
■airport at which he intends to arrive.
The connected flights must be detailed on the same ticket or conjunction tickets to qualify for the exemption. Tickets can only be regarded as conjunction tickets if:
a. they are in one booklet, or
b. where they are in separate booklets:
■each refers to the other and states that they are to be read in conjunction, or
■there is a summary of the flights constituting the passengers journey including the flights in question.
c. where the tickets are purchased online, they are purchased at the same time through the same portal.
Although the flights may meet all the other criteria for determining whether two flights are connected, they will only qualify for the exemption if the connection is evidenced on the ticket or a flight summary.
Dave