APD is due unless connecting (<24hrs) on one ticket or conjunction tickets.

The connected flights must be detailed on the same ticket or conjunction tickets to qualify for the exemption.
See: https://www.gov.uk/government/public...nected-flights

It has been discussed several times before:
http://www.flyertalk.com/forum/u-k-i...-question.html
http://www.flyertalk.com/forum/virgi...funded-va.html
http://www.flyertalk.com/forum/u-k-i...questions.html
http://www.flyertalk.com/forum/briti...-possible.html
http://www.flyertalk.com/forum/briti...m-transit.html
http://www.flyertalk.com/forum/qanta...pd-refund.html
http://www.flyertalk.com/forum/ameri...rture-tax.html