Each year Britain's airports rake in over £4 billion, and almost half of that doesn't come from flights, but from the hundreds of shops selling to the millions of passengers as they pass through the terminals. So what are the secrets of these mammoth businesses? For Dispatches, journalist Harry Wallop investigates just how Heathrow and other airports make their money. Going undercover, Wallop reveals the tricks we should all be prepared for and asks whether we are really getting a good deal from our biggest airports.