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.
That doesn't ring true.
The 24 million figure is
all transfer traffic, not merely international/international transfer passengers (the ones who don't pay APD). Although the proportion of transfer traffic that's domestic/international has been gradually falling over many years, it still accounts for around 20% of that figure, i.e about 5 million.
Heathrow: Facts and figures