In the fourth in our series on the world's busiest airport, we review an airport that has been more consistently in the news than any other; London Heathrow. For decades the world’s premier international connecting point, virtually every flag carrier on earth has wanted to serve it. For many years access to Heathrow proved the major stumbling block for agreements between the US and the EU. And, despite its pre-eminent position, the airport’s physical constraints have made it increasingly congested with any attempt at expansion beyond its present footprint endlessly debated and eventually denied.