[blam]

"Another thought, if the problem is with the "planes not the passengers" which was stated in previous posts, is it possible these specific flights are being targeted from the ground?"

I wonder if they have found some altimeter triggered devices on any of these cancelled flights? (I'm suprised this hasn't already been used, like the Lockerbie bombing)

[Lessismore]

The bomb was smuggled aboard the Lockerbie plane at Frankfurt, not Heathrow.

Most likely the common thread is not at Heathrow, but rather a concern about the airport(s) that the planes came from or the passenger list on the inbound flights.