I live in between the Ockham and Biggin holds for Heathrow and have a similar thing on the iPhone (Plane Finder).
When the ash cloud thing was all happening it was extremely accurate. As the flights were starting to arrive back at heathrow on that first night when Willie Walsh told the CAA to go take a flying @%#! and the following day, when I heard a plane overhead, I could see it right over my location on the Plane Finder map and tell where it had come from and what the passenger in seat 21J had for their meal.
These iPhones are amazing!!