In effect, of the 69m pax who used Heathrow in 2011, a staggering 24million transfer passengers fell into this very specific category and paid no APD at all. This massive loop hole allows Heathrow to benefit from what in effect is a significant hidden subsidy compared to other UK airports.
Or these lucky people avoid being slammed against a wall and having their pockets emptied by Gideon Osborne on the pretence of being green.