Apologies if this is rather off-topic, but I thought an explanation of APD here might be warranted.
Phileas - HMRC is rather more alert than one might have hoped.
If for example you fly Heathrow-Vienna-St Petersburg, and you have a connection time in Vienna of (essentially) under 24 hours, then the non-European rate of APD needs to be paid - i.e. £40 and not £10. Thus, unless you are prepared to spend more than 24 hours between 2 flights, whether you fly direct or not has no impact on the APD due.
It should also be noted that most airlines deem a break of 24 hours or more as a stopover, and will frequently charge a higher price for the privilege.
This alas means that if you want to fly to Russia / Ukraine, there is little or no point in choosing to fly via a 3rd country unless you wish to acti8vely spend some time in that 3rd country - you either pay more in tax, or you pay more to the airline.
You can find out more about the current scheme at
HM Revenue & Customs
The rules on what consitutes a connecting flights are described in sections 3 and 4
The APD scheme will of course change on 1 November 2009