Where's the most unionised square foot in Britain?

You might expect the answer to be "Barrow, where they make nuclear submarines", or, perhaps, a fire station somewhere - although the Fire Brigades Union is very small, its density is high. Or a school.

The answer, however, is Heathrow Airport - which is one reason why both the Trades Union Congress and Unite, Britain's largest trade union, have been so vocal in their support for expanding Heathrow (in addition, mechanisation means that 14,000 jobs on the existing site are considered "at risk").